Here's two easy to access resources. For US uniform info check out www.atthefront.com
,it's basically the Walmart of WWII US & German reenacting. Check out the photo references in the US section and they're usually pretty good about giving a basic history and construction information about each uniform product. Plus it'll give you a chance to learn the proper terms for each uniform item which will make it easier to research further. Also check their Materials section. After they finish a production run they usually sell their left over fabrics by the yard. They're the only place I'm aware of that does. And check their Links page. Atthefront has links to all kinds of resources and reenactment groups.
For WWII era Soviet stuff, I'd recommend getting in contact with the 197th Guard (www.197thguard.com)
. They are a WWII Soviet reenactment group that I've worked with many times in the past. They know their stuff. They might have a link or two on their website that might be helpful.
Sorry that's about all I can offer. Have fun at Costume-con, WWII stuff ought to fit in just fine there