Unless you are trying to get something exotic, like stainless or something, I would get the valves from the machine shop that is doing your heads.
You do need to pay close attention and get exactly the valves and stem length you need. There are any number of length and keeper groove height variations and locations on SBF valves. (Are you sure the stem length is .050 longer, and not just the keeper groove location?) Most of this stems from the use of the rotator assembly on the exhaust valves, making the stock valve grooves and springs different from the intakes, and sometimes, 351w's different from 302's.
There are also .050 offset valve keepers, etc etc, to fit this stuff together.
Otherwise, for just stock valve replacement, it would be best to let the shop do it all. I use the Speed Pro/Federal Mogul catalog for reference and shopping if I have to do it myself. Comp Cams tech help is also pretty good. The Ford Racing Catalog is a good reference, but you have to read it very carefully.