One important point to watch for is unit scale, which actually varies between unit types. There have been some estimates posted in the forum now and then, such as here:
http://www.axisandallies.org/forums/index.php?topic=34109.0
Because of these scale differences, it’s probably not possible to single-source miniatures from a single supplier, other than from companies like HBG which make units specifically intended for use in A&A. GHQ, if I recall correctly, makes WWII ship models which are too large for A&A; Navwar makes smaller ones, I think, but you may have to go with even smaller ships than those because they’re still meant for tabletop wargaming rather than the A&A board games.