basically any type of halogen flasher.. you could even jerry rig a turn signal flasher up to it if you were so inclined.. (and wanted to do it on the cheap)
I've also seen people use School Bus warning flashers since they are pretty much bulletproof, and have a very noticeable flash rate. (also heavy use rated, and can handle rather large amperage) Run about 9 bucks last I checked in Canada.
Got a spare HLF or TLF sitting around? There ya go as well..