The supermac Thunder II, Radius Thunder IV cards are decent. It all depends on what you are going to use the IIfx for. Video cards with DSPs onboard mostly just speed up photoshop, so if you are not a heavy photoshop user a cheap Thunder/24 would be great (and easy to find).

Some Rasterops cards were decent as well.

Apple 24*AC cards are a bit hard to find, and have limitations on what OS they work on.