I have only been satisfied with bones I make myself. I have a very nice set I made from maple and a larger set I made from yellow birch. There is no set size, there are no definitive guidelines except the the curve has to be wide enough for your fingers at the top and they have to meet towards the bottom. My maple set is 6 3/4 inches long and 1 inch wide. They meet at about 4 1/2 inches. The are about 3/16 inch thick. These are the bones I used when recording with Castlebay.

The yellow birch set are 7 1/2 inches long and 1 inch wide. They are 1/4 inch thick. These meet at about the 5 1/2 inch spot.

The yellow birch are louder, have a crisper sound, and are pitched lower but they are also slower. I can get faster runs using the other set.

I cut these on a bandsaw, cutting the curve into them. On my last run home I made a new set, also of maple, but I wanted to experiment with bending them into the correct curve. The experiment was inconclusive. I brought them home with me to keep trying. Maple is a very dense wood and it takes more steaming than I was able to do in a crowded kitchen.

I had a problem, early on, with the bones sliding through my fingers. I solved that by cutting a curve into the side of the bones to match the crotch where my two fingers meet. It works quite well.