They generally come in a set. I prefer brass over steel, less likely to leave marks. If you have a Bosch 1617, you'll need two adapters from their bayonet style to use the more commonly found PorterCable threaded type. Stick turned me on the very short sets that leave more of the bit exposed. Many of us use 1/4 MDF to make patterns with and the short versions, but I can't recall exactly where I found them, I think it was Woodcraft. For most pattern work, you will be using the bearing on the bit as your guide, not a bushing. The bearing will ride on the pattern.
The more I do, the less I accomplish.