The apparel industry is very consolidated just like other industries (food service comes to mind! You know-- all those "private label" items you see in the grocery store (Wal-Mart brand, "Archer Farms" (target), etc. Obviously Target does not have a ketchup manufacturing plant, a Cereal manufacturing plant, etc…)
Most apparel companies get their "blanks" from a very select few manufacturers. Overseas manufacturers in China, etc. may manufacture for hundreds of companies on a "contract" basis, no matter what the tags say. These brands simply contract with large manufacturers to make the clothing to their specs. Try searching sites like
www.alibaba.com and others to find trade partners in China, but beware there are many foreign companies that say they're manufacturing your stuff but they are really using another company! In the end it's a few manufacturers feeding the entire supply chain, just like most other industries!
One example:
Manufacturer in China > Chinese Broker > Import Broker > USA based “wholesale company” > Smaller Wholesale Operation > Screen Printer > Consumer