It’s not that easy... Just because you can buy something doesn’t mean it’s legal to possess.
Our laws are inconsistent and difficult for even US Citizens to follow. Laws concerning pepper spray can vary by State, County and city. New York, New Jersey and I’m sure other States regulate their use, possession and the strength. Stun guns, tasers and knives are regulated even further. About the only defensive tool you can legally carry almost everywhere is a sturdy cane and you better be able to fake a limp!
Study up on the places you plan on visiting. Use common sense and stay away from high crime areas, especially at night. The USA isn’t as dangerous as you may have heard, but there are places I simply wont go; even in the middle of the day. You can buy wasp and hornet spray almost anywhere, but never admit you have it for defensive purposes.