Brownell's sells carding wheels. They aren't particularly cheap - $70-80. They're nothing more than ultra-fine steel bristles on a wire wheel.
Frankly, I mainly use 0000 steel wool for my rust bluing, the oil-free variety to prevent oil intrusion into the process which must adhere to squeaky clean protocols. You won't find oil-free steel wool at the hardware store, the stuff is oiled to prevent rusting while sitting waiting to be sold. I buy it on line, more expensive than the hardware store variety but still cheaper than a carding wheel. When carding with either method it's highly advisable to wear a face mask to prevent inhaling the airborne particles.
Two kinds of iron oxide (rust): ferrous oxide which is black, and ferric oxide which is red/brown. Ferric oxide needs to be converted to ferrous oxide which is essentially the blue/black we admire on our steel guns, hence the boiling in water. (Don't ask me to explain the molecular chemistry!) The rust bluing process is essentially creating a thin evenly controlled coating of red rust, then converting it to ferrous oxide, then carding it to leave behind a blue/black "stain" on the steel.
The only issue with boiling old rusty gun parts lies in the red rust not having been evenly created under carefully controlled circumstances, with the attendant risk of uneven pitting a likelihood. Still and all I see it as being a beneficial process, way better than having a sorry mess of a gun.
