You can buy different forms of it, such as strontium citrate, in supplements at supermarkets and health food stores.
Scientists haven’t tested these supplements much, so there’s not enough research to know if they fight osteoporosis. Also, the FDA doesn’t regulate supplements in the same way as prescription drugs, so it's not possible to know if if they are safe or work well or even how much of the main ingredient they contain.
If you want to take a strontium supplement, ask your doctor first.