I used to get screw kits from Rc Screwz for my other cars but have not been to their site for sometime.
They used supply kits for most kits, however, there are few things that will save you some money and one of them, it that , if they do not have the kit made up for the car kit then you can submit the specs and they will give you a discount for letting them know the specs.
I went to what I thought was there website, however, it appears to be down right now and not working properly.
Perhaps you could try some keywords and see if you can find them.
I noticed that Losi still advertises their products, so it appears that they still have sets for different types of cars, trucks etc....
Boca Bearings does the same type of thing if your interested in getting bearing kits made up. You just have to let them know all of the different sizes and they will build kits for all.
The address I have for the screw kits is http://www.RCScrewz.com
try it, or perhaps you can find a link to a new site they may have ????