Do you mean an Indivision AGA or a BlizzardVision PPC? Because unless you towerize the thing and go with a Mediator-type option, those are the only two cards for a desktop A1200. If the former, AmigaKit. If the latter, ebay. Good luck on that one, though! :p
Edit: I suppose some people would consider a DCTV to be a graphics card, that would also work with an A1200... LOL. Depends what you want to do though? Run CGX/P96 and play RTG games, or just use a modern monitor, or be able to display 24-bit images?