Neo Geo cables
There's the crusty RF cable, composite, and in Europe only (officially anyway, i beleive) RGB. If you have an RGB monitor or tv, you could get custom leads. Oh, and just yer common S-Video cable if you've had it modded for that
I'm sure someone will post the best places to try soon - ebay's worth looking and of course the neo store on your left
However, if you're a techy head you could make one yourself - check out <a href="http://www.gamesx.com" target="_blank">www.gamesx.com</a> for pinouts.