Hack a "PSOne" dual shock pad. That is the best compatibility around, and gamestop still has em for $5 used. I recommend the H-Series, but if you can find an A-Series with the ribbon connector, thats even easier to hack (ala SpiffyShoe's instructions).
I forget which GameCube adapter I bought, but its super cheap and really small...and works fine with the PSOne pad (which uses digital buttons).