A shop will probably want to sell you a whole new wheel. It'll be as cheap as rebuilding the old wheel (which is what replacing all the nipples amounts to) and it's quicker for you and them.

Besides, there's some possibility that you won't be able to find nipples that fit your old spokes. If the bike's Japanese (you didn't say what kind of bike it is), I'll bet newer DT, Wheelsmith, or Sapim nipples won't fit perfectly. Ill-fitting spoke nipples are a recipe for trouble.