hmm. About a mile down the road from my place is a small meat cutting operation. The guy processes deer and wild hogs during the season, but throughout the year his main business is slaughtering and butchering cattle and hogs for the local farmers' own use. It is strickly a small time operation.
All I've had to do is ask, and I can get a horn or two from him. But, of course, I have to let them dry for a year or so before I can use them.
I have to saw them off the skull, let them dry a couple of months, then boil them to loosen the bone core and remove it. Then, there is still a lot of pith (or whatever you call it) inside the horns that can be difficult to clean out.
I only need the occassional horn for my own use, or sometimes I'll make a powderhorn for a friend or relative, so I don't need many. I currently have 4 or 5 drying out in a shed.
The same guy will sell me a raw hide, too, for a reasonable price, if I want them, although he has a regular buyer for the hides.
Find such a small time operation near you. It don't hurt to ask.