These are sites with a variety of gas or electric motors for bikes, or motorized bikes:
bikemotor.com This is a roller type gas drive system, using several of either common 2 or 4 cycle motors. Reasonable prices, good variety, claim can retrofit any bike or offer several reasonably-priced models. DOT street legal.
Bionx.ca Electric bikes from Quebec, about $1,300. Regenerative technology; price doesn't include the bike.
bikeengines.com This set up was written about in Popular Mechanics here and also a pretty good blogger review, here. It has a superior belt-drive system (which allows for knobby tires), and prices for the motors and kits are generally below $600 - $700 (depending upon motor selected).