TAKE NOTE DUDES W/ PLAYSTATION HACKS. This is very good 411!!!!!
Playstation gamepad hacks are pretty easy since they have a common ground.
I suggest the latest Dualshock 1 by Sony (the white one, series H) You can desolder the digital boards ribbon cable and then solder your 16 wires (very tight solder job, for advanced dudes only!) to where the ribbon cable was. You must wire up a mode button and then you have a stick that can even work for USB, Dreamcast, Gamecube & Xbox with the appropriate adaptors. Email me for where to buy those adaptors.
This is a very straight forward hack, if you have soldering skillz...
Wiring code (top of ribbon, away from the LED)
1 mode change
2 Joy left
3 Joy down
4 Joy right
5 Joy up
6 Start
7 Select
8 L2
9 R2
10 L1
11 R1
12 Tri button
13 O button
14 X button
15 Sqr button
16 Ground
Can't hang? Paypal me $40 and I'll ship you one all wired up and ready to go. Info here is free, labor costs...