This is quite a common question people ask me when seeing my work at exhibitions or fairs. Well, I have several ways to collect bones. Sheep bones I find in remote uphill areas where farmers are allowed to leave dead sheep. Normally a farmer needs to call an agency for the pickup of dead animals. Over the last two years, I have learnt where good spots are where sheep might have died over the winter. I leave them there until all flesh is gone, takes about 6-10 months.
All the other bones, like rabbits and birds, I collect mainly beside the road. If I spot a dead animal, preferably not one that is hit full on, because lots of bones are break, then I either leave them there or move them to a better spot to let it rot. Also for collection later. A rabbit can be fleshless between 4-10 weeks, but you will have to get rid of some skin and fur yourself.
Sometimes I find something that is clean already. Once I found a ram skull completely clean while I was mountain biking. I attached it to my Camelback and continued my ride...probably with people thinking I was mad. Well, I am!
Another time riding on my bike, I spotted a deer that was hit by a car. I took two plastic bottles to put over the rear legs and grabbed it there and lifted it over a dry stone wall. I had to hold it far away from my chest because it was dripping blood, which was quite heavy. It is still there and would mean a full set of deer bones!
Today I went out and there were lots of interesting roadkill, but unfortunately didn't have much time to secure them. But two quite rare finds I did secure, one was a female grouse and the other a Stout (Mustela Erminea). The Grouse I had spotted earlier, but didn't have any equipment with me to move it. On the way back to it I found an almost complete hare skeleton 95% clean, that's home now. Then I found a fresh rabbit, collected the Grouse and found later the Stout. Very efficient bike ride!