Very common. They come with two and three cutters. Buy the ones with three. I bought a set with two cutters and took them back promptly - way to much vibration to control the cut.
Jet and Veritas make them in sets of 8 bits - 4 cut tapered plugs and 4 cut cylinderical plugs. sizes range from 1/4" to 1/2" (I believe, without running upstairs to the tool box). Jet costs about $28/set and Vertas about
30 to 50% more. Some stores sell individually.
As others have noted the real value of these, over dowels, is they allow you to cut plugs from similar wood as your project and with face grain (not end grain) exposed. They won't make the screw holes disappear but they can come pretty close if plugs are installed with care.