I'm lucky in that regard.
I have a pen back in the pasture where I drag carcasses, etc. It is crawling with those beetles the guys use to clean skulls.
I just put something out there, and go get it in a couple of months, wash it off, maybe bleach it a bit, and I have a ready to go skull.
A friend of mine who has the skull beetles didn't believe me, so I took him out back and showed him... He was shocked that I had the same bugs out there that he paid for and cultivated to clean skulls.
here's a hog skull I retrieved from my bone pen...