For god's sake man!
ANY BLOODY GEFORCE OR RADEON WILL DO!
Search ebay for "geforce" or "radeon", filter for anything with svideo-out and within your price bracket, and start bidding. You only want 2D, so the 3D capabilities don't matter.
Nvidia cards:
GeForce 256 SDR, GeForce 256 DDR, GeForce2, GeForce2 GTS, GeForce2 Pro, GeForce2 Ti, GeForce2 MX, GeForce2 MX400, GeForce2 MX440, GeForce3, GeForce3 Ti200, GeForce3 Ti500, GeForce4 Ti4200, GeForce4 Ti4400, GeForce4 Ti4600, GeForce4MX, GeForce4 MX4000, GeForceFX5200, GeForceFX5200SE, GeForce FX 5600, GeForceFX5600SE, GeForceFX5700, GeForceFX5700LE, GeForceFX5700SE, GeForceFX5700Ultra, GeForceFX5900m GeForceFX5900SE, GeForceFX5900 Ultra, GeForce6200, GeForce6600, GeForce6800GT, GeForce6600 Ultra, GeForce6800, GeForce6800GT, GeForce6800 Ultra.
And that's JUST in AGP. And probably another 20 I've missed. Ditto for ATi and all of their radeon family.
ALL OF THESE WILL PLAY MAME JUST FINE.
So, once more, search ebay for "GeForce" or "Radeon", check if they have S-Video (which is card manufacturer specific, not chipset specific) and if the card is in your price range, then buy it. Simple as that.
Telling you to go and find a "GeForce2MX" won't help, as there were three dozen different card manufacturers for that chipset, some of which had s-video and some that didn't!