I go to my grocery store to get meat or to my butcher. Kits can eat small bones like chicken wing tips. They can also eat adult mice, rats and quail and rabbit bones too. I get whole frozen mice from a small local pet store. Try looking for a place with reptiles and snakes as they feed mice to them.
You can use chicken with bone, quail with bone, turkey necks and backs, beef meat as the bones are too big, frog legs if you can find them, rabbit, adult mice, rats and guinea pigs. pork meat unless you chop up the bone in there. Probably duck too. Look to see what you have available locally in the way of meat. Some people will get grinds and freeze dried raw. I have never ordered raw online. I have a local pet store that sells raw ground meats but my ferret didn't like it. I also wanted him eating bones to help clean his teeth and ground meet doesn't seem to do that.
Kits can eat just about any bone that adults can as Charlie said, although they will have to build up their jaw strength first. Gizzards are good for that. I didn't have much luck with turkey necks, personally--I found them too difficult to portion out and I found I had to break the vertebrae with a mallet for Rocky to eat them. Maybe they're just too big in my area though.
Really, any animal protein is appropriate, just be sure to have a minimum of 3-4 bone-in proteins per week. Fish should only be fed occasionally as it will cause them to smell (and IME they tend not to care for it much anyhow). Just be sure that any cuts you buy from the grocery do not have any added salt or spices.
Pet stores will sometimes have frozen mice for sale in the reptile section, although it's about double the price of buying in bulk online.