RE: nitro mini rs4
Ahh, the HPi nitro mini! Still one of my favorite cars.
HPi themselves made universals for that car, and depending on whether or not you feel like ordering from over seas, they are still available from here [link=http://www.rcmart.com/catalog/for-hpi-hpi-rs4-mini-c-595_737_865.html]http://www.rcmart.com/catalog/for-hpi-hpi-rs4-mini-c-595_737_865.html[/link]
I've bought stuff from RC Mart numerous times without issues. In fact thats were I bought my set of universals from.
Also, MIP made CVD's for these vehicles, they are hard to find now but they do pop up on Ebay from time to time (if I don't beat you too them, lol) Really the only parts you need are the actual short bones for the mini, the axle pieces are fom the standard RS4 and the pins and sleeves are all the same for 1/10 scale, I have seen just the bones for sale on ebay as well.
Aside from that I don't know of any other ones that fit.