You go to the model you have and click on option parts or stock parts and then can buy what you need. I find most of the stuff is priced the same as my local Hobby Town, but don't know what shipping is like. I just bought a Savage X trans case/center bulkhead and high-speed 3rd gear from my lhs and it was the same price as on the HPI site.
Even eBay, for the stock replacement stuff is about the same unless it is a killer deal. That high-speed 3rd gear, with shipping on eBay is about identical to my lhs with local tax and I get it NOW.
HTH...
Mark