Apparel was actually one of my specialties for several years during/after college.
The best approach to getting products really depends on what kind of clothing site you're trying to run. I've covered a couple of the more common scenarios below...
Trendy boutique e-commerce site with brand name stuff
-Go to similar brick and mortar stores and check the tags. Contact companies directly. You'll need a proper business license/tax ID for most, and some will not want to sell to you if you're strictly e-commerce.
-Check out apparel and accesory shows. Just Google "apparel show" and you'll find plenty.
-If you're looking to print up shirts/yoga pants/etc, check out American Apparel and Alternative Apparel. Both work with smaller buyers. Also, most screenprinting places have a huge catalog of basic pieces they can print on, and most include nicer stuff, too (organic cotton, bamboo, weird athletic fabrics, etc.)
-Most "real" brands don't put themselves on wholesale sites, unless you're talking about the sites owned by industry-specific publications.
-Check Etsy.com and approach designers directly to work out a deal for bulk purchase.
-Alibaba.com (and a couple similar sites like HKTDC.com) can be good, especially if you want to create your own brand - but quantities are often larger, there are quite a few scammers, and it can take a long time to work out a deal, get it shipped, get it through customs, etc.
Closeout store with limited quantities of brand name stuff, usually a season or two old
-Closeoutcentral.com (plus a lot of the sites mentioned so far) - I had really good luck with a few closeout vendors, but I also got a few bad lots from the same sources. Some places cherry-pick their lots and give you a lot of junk. In general, anyone who will give you a list of what's included will be more expensive. Even moreso if they offer pictures. Also, if it's closeout stuff expect gross stuff like deodorant stains, makeup around collars, etc. from time to time. I don't know if you have a preference one way or another about how many of each item would be in a lot, but you'll turn over your available cash much faster if you make sure to get lots that have a lot of variety. If pieces are the same, you generally want different sizes instead of 20 blue blazers in size medium. I didn't give much thought to this on my first lot. It made my job easier in that I didn't have to create 20 different listings for 20 different items...but it also slowed down my ability to earn a profit on that money and reinvest it into more lots. I did have a website that I used to sell clothing, but I also used eBay a LOT for advertising purposes and to do more volume overall.
-HotTopic.com and other e-commerce stores - This may sound weird, but I got a TON of stuff from sales at other stores. On average, I did a lot better with stuff I bought from retail stores because the prices were pretty similar to the prices I'd get from closeout dealers...but I would be getting it BEFORE it hit those sources, so it was easier to get stuff that didn't totally suck, and you would get it while it was still newer (crucial for anything trendy). I watched sites like Slickdeals.net and pounced the moment there was a decent sale. I'd get $20-25 graphic tees for $3-5 and resell them pretty easily. Same for hoodies, thermal shirts, shoes, accessories, etc. I mention Hot Topic specifically because every so often, they have a 50% off clearance sale. I loved those because I could generally expect to triple my money. Puma's online store is also known for really good sales where you can stock up.
-TJ Maxx, Marshalls, Ross, etc. - These kinds of companies are some of the biggest buyers of closeout apparel, and a lot of manufacturers and big stores sell directly to them (meaning they get stuff that would never make it to the closeout resellers you'd find on directory sites). I used to drive all around St. Louis to hit every one of their stores, then come home with thousands of dollars of merchandise that I'd re-sell either on eBay or my own site (and eBay was great marketing for my site). Department stores and normal retailers occasionally have a lot of good stuff on clearance, too (at prices comparable to what you'd pay wholesale if you shop well).
-Google - If you have specific brands in mind, remember that they often restrict closeout places from stating the brand. Some get around this by abbreviating brand names or adding in a dash or two in place of certain letters. Once you browse some wholesale sites, you'll see the most common ways this is done and you can Google those phrases. "Famous maker" is a common term used, too. Try searching "famous maker" wholesale (and add/exclude additional words as needed).
Other random stuff...
-http://www.apparelnews.net
-When you find a site you like, use Ahrefs to find out who links to it. Often, you'll see links from resource pages that include other sources that might be of interest to you.
-Information on retagging and relabeling for your own brand -
T-Shirt Tag Relabeling and Finishing - T-Shirt Forums
-http://www.primetimeclothing.com
