The size is probably important, although to be honets, a couple of lmillimetres heare and there isn't going to break anything.
However, Em-4 Miniatures make 30 and 40 mm flat round bases (same style as the GW ones):
http://www.em4miniatures.com/acatalog/MINIAATURES_BASES.htmlIf you find anyone who makes 50mm ones, give me a shout.