I re-shafted a set of irons for a friend of mine and used Apollo Standard iron shafts. He hits them pretty well and hasn't had any complaints. I haven't used them myself though.
I have the Apollo Stepless Shafts cut to stiff in mine. I love them I can't tell the difference between them and the DG S300s I had in my old irons. Great economy shafts.
Just to throw out another low cost alternative, you might want to look at the FST shafts. They're just as economical as Apollo and, I think, have much better consistent quality than Apollo.