I bought one from a member here. It works pretty well. He order it from:
http://www.tototek.com/
They have "Joypad converter for NeoGeo"
Product Description
Now you can use your lovely psx joy-pad, dual-stock joy-pad or dual-stock 2 joy-pad run on your NEO-GEO console with your favorite games
* 2 levels of Turbo Fire on A, B, C and D buttons
* Slow Motion
* Analog function supported : You can use Analog stick to replace Up, down, left and right buttons.
* No External power supply needed. It's use console internal power.
* Never hurt your fingers, because it's use PSX/PSX2 joy-pad, This is more comfortable feel as good as Playstation2.