Summit and probably other places have sets and individule bits you can get. I found some at NAPA.
I didn't like the square end bits bacause they tend to dig in if your not paying real close attention.
I also made a sanding bit.
Take a long 1/4" drill bit (cut the flutes off) or 1/4"steel rod and cut a slot 3/4"-1" down the length of the bit. You insert folded sand paper into the slot and now the inside of the ports can be sanded to the finish you desire.
I'll get a pic tonight if needed.
The 1/4" bit or rod can fit in your drill or die grinder.