Got my answer. WCL

Ok, My Dad would use Rock salt if that is another idea for you. He said you just have to clean your barel real good after you shoot.

Myself I wouldn't think buckshot would be hard to find anywhere, try Walmart.
I get 12 guage rubber slugs and bangers and flares from a specialty store in Alberta.
They also sell electric fencing etc. Call your game branch for ideas.

Unless you have properauthority to do so you may end up defending yourself in court for shooting wildlife with rubber bullets. Most states have specific laws that make things like this illegal.