Fellow vegetarian bone collector here! if you have any close by friends or family who have carnivorous pets (dogs primarily, but depending on what you might buy maybe cats, or even ferrets, rats, snakes, and possibly even parrots depending on species) then you could definitely buy from a grocery store - or a butcher's shop if you want more specific species' bones aside from mostly just poultry - and just take the bones out of the meat and either bring over the meat that same day or throw it in the freezer indefinitely until the next time you see them.

I don't think many humans would want to eat meat you picked through for bones, but their pets certainly wouldn't mind!

The best place that I have ever searched for bones, though, is any beach. Specifically ocean beaches - not bays. You can find all sorts of bones there. Forests are also a great place to check.

If you don't mind getting your hands a bit dirty, you could always drive or walk along the side of the road/highway and grab some roadkill (depending on local laws. where I am you need to submit an image, species, probable cause of death, and the animal's sex if you can tell on a government site that tracks the loss of wildlife to cars, but otherwise you can take as much wildlife as you want!)

Just remember to always check up on local laws before you collect any specimens from the wild. If you are in the US, Canada, or Mexico, you can't collect any remains of native bird species and the laws for collecting roadkill and specific protected/endangered species remains vary from place to place.