Here's what the rather brilliant website www.nycpulse.com says on the topic. My first stop (for pretty much everything in Manhattan) would be Bloomingdales, mentioned below. But you might want to try something quirkier - here's the Pulse's (rather long) overview:

"Andy's Chee-pees
691 Broadway (bet 3rd & 4th St); 212-420-5980
This store located on Broadway attracts a ton of foot traffic from tourists drawn by the window display. A few vintage odds and ends (the usual old Ts, jeans, jackets and a bunch of cowboy boots) are scattered in between the new merchandise, which includes Emily Strange, Dickies, French Kitty, new retro bowling shirts, and kitschy purses. If it’s vintage you want, it’s vintage you’ll get at Andy’s, but be aware that most of their merch is brand-new.

Barneys New York
660 Madison Ave (at 61st St.); 212-826-8900
Habitual shopper or mere impulse buyer, Barney's is the place for the final word in trendy yet timeless designer fashion. Within your first footsteps, traditional diamonds and nouveau hip jewels surround your senses. Just a few feet further and you'll find the latest and greatest of Marc Jacobs carryalls. Shopping for the fashion-conscious man in your life? You'll find the perfect selection of crisp, collared shirts and signature dress shoes. Fear Barney's welcomes only those with a black Amex? Don't let them fool you. Although best known for their pricey couture, Barneys is constantly equipped with exceptional sales. Find an off-the-shoulder Ella Moss long-sleeve on sale for $49, or a tastefully bold Hugo Boss tie, a mere $44. Whatever you're shopping needs, Barney's has what you're after.

Big Drop
1321 Third Ave. (near 75th St.); 212-988-3344
Although leg room is definitely at a minimum, the shopping at Big Drop is no less than first class. Truth be told, Big Drop is another “ladies only” venue (sorry guys, you’ll have to fly elsewhere). Golden Hawk satin and lace spaghetti strapped tops are the perfect accent for the hip hugging Frankie B jeans that Big Drop also has to offer. Want to look good without looking like you’re trying to? Shopping at Big Drop will make it look effortless.

Bloomingdales
1000 Third Ave. (at 59th St.); 212-705-2000
Want to look the part without having to break into your life savings? Without hesitation, go to Bloomingdales. Hosting the best selection of the biggest names for the best value, Bloomies takes the gold in the department store category. Although at first you may find yourself plowing through the crowded main-floor, like a breath of fresh air, the women’s department in only a staircase away. You’ll find the widest array of designers that are somehow…always on sale. For women, find the best of James Perse and Rebecca Taylor. For men, from Sean John to Ralph Lauren...there are clothes for any mood or moment. Where better to refill your wardrobe and redecorate your house in one felt swoop?

Century 21
22 Cortlandt St. (near Broadway); 212-227-9092
Although the constant hoards of shoppers and overflowing clothing racks may leave your Century 21 shopping experience with a little something to be desired, you simply cannot beat the hit or miss (but guaranteed to hit at LEAST once) euphoria that comes along with this enormous downtown discount bonanza. Although a little elbow grease may be necessary, find a mouth watering selection of last season Marc Jacobs and the best offering of discounted Rebecca Taylor that New York City has to offer. Looking to be a little different this fall? Hop into a pair of Rebecca’s unconventionally adorable ankle length pinstriped pants. A perfect substitution when your MJ classic stove pipe jeans are in the wash (also found here). Guys, you won’t leave without a pair of $50 Seven jeans for yourself. Not to mention department after department of home décor…perhaps you should see for yourself!
Diesel
770 Lexington Ave. (at 60th St.); 212-308-0055
Euro-chic might be the way I’d choose to describe the funky style that Diesel exudes. Undoubtedly the winner for the most artistic and creative merchandise displays, Diesel’s clothes host the same artistic quality. Find the perfect men’s “already worn” tee for around $50. Ladies, find jeans to ride on any size hips without feeling like you’re wearing the same denim as everyone else. You’ll find endless variations of their signature styles for anywhere between $80 and $200.

GAP
1511 Third Ave. (at 85th St.); 212-794-5781
Affordable yet “classically” trendy, you simply cannot go wrong at the GAP. Any man or woman will have no trouble finding that perfect pair of jeans made to mold to his or her individual curves. Chock full of staple items for your closet, the GAP has it all. Find fashions for teens (silk screened tees, around $10), for men (the most comfortable pair of army green corduroy pants, around $50) and of course, for women (the most flattering pair of “Long and Lean” jeans in an array of different washes, around $70). The best part about it, you’ll find it all under one roof. Season after season, you can count on GAP basics to fill your wardrobe. Ladies, try a four season, buttery soft denim skirt on for size (around $25). Just “fall into it.”

Helmut Lang
80 Greene St. (near Spring St.); 212-925-7214
You’ll feel a bit like you’re walking into an art gallery when you step into the doors of Soho’s Helmut Lang boutique. For both men and women alike, find a truly upscale line of denim designers (up to around $250). Expensive? Maybe a tad. But well worth it… Lang never lets style exceed the boundaries of comfort. Looking for a get-up to lounge in? Helmut Lang is home to a great selection of Three Dots and Hard Tail cotton knits, sweat shirts and sweat pants. Be casual and chic at the same time…how perfect.

Ina
208 E. 73rd St. (near Second Ave.); 212-249-0014
Jimmy Choo for under $200? You can’t beat that with a stick. Find treasures for less than half of their original (and perhaps daunting) prices in this quaint village-esque uptown consignment boutique. Deterred by the word “consignment”? Well, don’t be. Ina’s top notch styles are nothing to sneeze at. Putting quality way above quantity, Ina fills her petite boutique with styles by haute couture labels such as Gucci, Prada, Burberry and even (yes, it’s true) Louis Vuitton. I shall not tell a lie. Ina is by far, one of Manhattan’s best kept shopping secrets.

Levi's
750 Lexington Ave. (near 59th St.); 212-838-2188
Expecting just a pair of plain old 501 jeans at the Levi’s store is quite the misconception. A perfectly hemmed stone-washed denim skirt won me over for a mere $35. Unbeatable? Levi’s is definitely in the running. The selection of men’s denim is undoubtedly the store’s best attribute. So, if you’re shopping for the man in your life (or you ARE that man) give the classic American look a try at Levi’s. Offering endless fits and washes, boot-cut or straight legged, relaxed fit, or low rise, you’re bound to find what you’re looking for."