Depends on what your tv has. If it has component input, basically any card with dvi output will work with an adapter. If it has s-video or composite input, any card with s-video out will work (you'll need an adapter if it only has composite). You'll probably want to go cheap if you are just using it for MAME as it relies mostly on cpu not graphics card.
Component > s-video > composite
Newegg.com usually has the best prices. Other places to look would be TigerDirect.com or your usual stores like Best Buy or Circuit City, although you'll probably be able to find a less expensive, more suited to your application, card online.