Try a guy on ebay called jr-rc he breaks kits up and sells the parts, I got a full bearing set off him for about 4 quid
Also if you're after hardened diff oudrives that don't break up, look on the 3racing web site, I ordered 2 full sets off them at the same time as my chassis and towers, they're a lot nicer than the tamiya stock ones and are the same type front and rear (although still specific to each end) and have lightening holes milled in them.
-Steve