Are you talking about the factory black ones? You will find them on sale here from time to time, or Craigslist. I bought mine from a member here for $100 local pick-up, you will sometimes find deals like that.
Depends on your budget, if I was to buy and I couldn't find any used, I would check out American Trucks's website, they have a lot of running boards at all kinds of budgets. Put your truck specs in there and see what's available that you like.