If you search for '28mm ww2' on ebay there's quite a lot of them but of course it's not all in the U.S.
http://www.brigadegames.com/ might have some but the page is a pain to navigate.
http://www.theassaultgroup.co.uk/ has all sorts of modern and historical 28mm models, but is in the UK.
Check out this topic: http://www.deathsquadsgame.com/general-interest-f16/alternative-models-and-vehicles-t108.htm
You could be lucky