Anything type of plastic container will work, you just need to cut drainage holes in them. I have used old yogurt containers, chinese take-out soup containers, V8 bottles with the tops cut off.....the list goes on and on.
You can also put rocks in the bottom of the pot to help weigh it down. Then put the medium on top of the rocks.
Or you can put the plastic pot inside of a clay pot to keep it from tipping.