Just spotted this thread mate, sorry for the late reply! There are two hardened steel diffs for the TamTechs. The first is made by Atlas (#OP8-223) and has been perfect since fitting it in mine. I got mine from ebay but I haven't seen them for ages. Dinball are still showing Atlas parts listed so might be worth starting there.
The second is made by 3Racing and is actually specified as a GT01 diff but I am pretty certain they use the same parts so it should be a direct fit in the GB01. This again is available direct from 3Racing or you should get this from places like Dinball or other big online stores (all abroad though). I doubt you will find anything other than the aluminum hop up in the UK. The ali diff is pretty good too though, just wont last as long as the steel versions.
http://www.miwahobby.co.jp/eindex.html Team Atlas
http://os.3racing.hk/products.php?products_key=2377 3Racing