If you are referring to Bullnose DRYWALL corner bead, just go and buy it. H.D. carries it. There are actually different radius sizes that you can get, at drywall supply places.
If that is what you want (drywall bullnose), then the option is to just purchase it.
I posted the same question a while back, and I found no magic solution. Back to the premade corner store I went. I was actually trying to miter a bullnose edge around a window with a kerf. The only premade corners I found had a "3rd leg" that I just cut off so it fit into the kerf.