Try Elheim. They have similar heft and a pretty wide (and expanding) range. I've been able to mix some Elheim with GDW Twilight 2000 on the tabletop without difficulty.
Also, I've used Combat Miniatures, currently produced by Stonewallfigures.co.uk and they mix quite well with the GDW Twilight 2000 range.
Platoon 20 and Liberation Miniatures, on the other hand, don't mix all that well, nor do Stan Johansen Miniatures – they're all too short and chunky.
I do like the vehicles offered by S&S Miniatures, but I don't think their infantry miniatures are sufficiently detailed to mix. Other sources of vehicles are pretty much any 1:72 die cast such as Dragon, Altaya, Trumpeter, Easy Model, etc.
Check out Guild Wargamers for lots of 20mm discussion and news boards for manufacturers. link
Hope this helps.
begins to think about terrain for Team Yankee.
