I got my joysticks and buttons from xgaming, since they were cheap. Thier uk office doesn't supply these parts yet, but if you email the american office they will charge $20 postage which isn't too bad (their website states it will be $60!) US based.
www.xgaming.comUltimarc is also a good bet, their buttons and joysticks seem to be highly regarded on here. Uk based.
www.ultimarc.comI also found a company called Swallow Amusements, which sell parts. I haven't dealt with them so I can't give any advice. Uk based.
www.coin.demon.co.ukYou can get tmolding from tmolding.com, they're american but the postage isn't too bad. US based.
www.tmolding.com