Every online store that handles chassis parts for racing or 4wheeling sells heim joints and various suspension joints. The list includes Poly Performance, Spidertrax, Ballistic Fabrication, and on and on and on. I recently saw a special on 1" heim joints and misalignment spacers at Dirt-Fab. As far as bolt size, 9/16" is adequate, 5/8" is better and plenty strong enough. Obviously they should all be grade 8, torqued to spec.
Joint size depends on what you want, and what you want to spend. For many a 7/8" heim joint is strong enough, but whatever yo get needs to be high quality. A high quality 3/4" joint is better than a moderate quality 7/8" joint. Personally, my buggy front 3 link has 1 1/4" heim joints on the lowers and 1" joints on the upper. If you want to save a few bucks, then use johnny joints rather than heim joints.