I just replaced the power supply on my NEO GEO AEC, a Euro unit. I used a Maplin product, the Maplin L48BQ 3600mA Switched Mode Traveller Power Adapter. It comes with a variety of plugs and it has one for the Neo Geo. This was hard to find because of the current draw required by the NEO GEO. The Neo uses a tip NEGATIVE plug and I used 7.5V to get it to work. I tried 5V and 6V but I couldn't get a steady picture. The Maplin unit works with 100-240V input voltage and produces 3.6Ma at 5V, 6V, and 7.5V. 3100mA at 9V. 2300Ma at 12V and 1900 at 15V. Maximum current out is 28.5VA. It's a cheap unit also at 30 euro ish.
Anyway, works a treat.