I'd get them from Van's. Here's why: tolerances could come into play.
I had rivets with my kit that were from two different manufacturers. Most fit perfectly. The minority were a little larger in diameter, though still within spec, but they did not fit and the holes had to be reamed a tiny bit. Not a show stopper, but a very annoying hassle. Several builders posted here awhile back with the same complaint, so it wasn't an isolated case.
If you buy the rivets from Van's you'll at least have the ear of somone who cares if there's any problem.