Most cactus...that is, actual cactus and NOT succulents, are edible. Mainly we feed the spineless opuntia varieties. Some succulents are toxic and you'd need to know which kind before you feed them.
You can buy opuntia in the produce section of any hispanic store. My store (Not an hispanic store) sometimes stocks it. It is called nopales or if it's cut up, nopalitas.
This is what the plant looks like:
And this is what it looks like in your grocery store: