In addition to the commercial ones mentioned, there are many homemade solutions to the stability of canister stoves, such as 3 titanium shepherd hook stakes (which most of us carry for our shelters), 3 halves of wooden clothespins plus a rubber band. I've never seen the need for any. I want my stove on bare ground anyway to avoid catching any dry vegetation on fire, so if the surface is sloping I generally level the ground with a stick, replacing the surface dirt removed afterwards.

Level ground is necessary anyway, whether or not the canister is stabilized, because if the burner surface is slanted the pot will slide off. This inevitably happens just when your water is just starting to boil!