Now, I don't collect them myself yet but am planning to do so as well.
As far as I heard there are a few options on how to get your hands on bones.
I have read that you could try asking at local museums if they have old specimens they are willing to sell or give away.
Other than that:
- flea markets
- Vintage Stores
- Online Stores/Etsy
- Hunting clubs (Not sure if these exist in the same way as they do here in Europe. I can only speak from the options available to me here, sorry!)
If you are willing to prep them yourself I advise going to any local forrest and searching for owl pellets. Oftentimes there are Mouse bones or the like in there.
Or you can go to the next butcher near you and ask if they have bones, skulls etc.
However that process is quite difficult and time consuming.
I hope this helps :)