Get a set of Kart hubs and used kart wheels. The hubs you can spend what you want. The cheap ones are better than stock wheels. The ones I had on my SP were $20 if I remember and they lasted forever.
Wheels can be found used cheap. Find a kart shop, they will probably have some laying around or will know of some one who will.
I don't know of any 6" steel wheels that will work on the 3 bolt hubs.
Using a hub and wheel set up is the best way. That way you can replace each individually if you break or bend one.