Research Resources: 20th Century US Military
However, the internet is filled with resources. Here are a few of the links I used when working on WWII Army Air Corps uniforms.
Garcia Aviation - World War II Reproductions and Originals for sale
U.S. Army Patches
WPG Militaria - World War II Reproductions for sale...bonus you can buy Indiana Jones gear!
ETO Militaria 39-45 - World War II Uniform and Equipment information
Aviation Wings and Badges of WWII
Olive Drab - Military history from WWII to Vietnam