Paintball Co2 O ring question?

So I recently got into paintball, and have an Empire 20 oz co2 tank. It says it's "Propelled by Pure Energy" so I assume Pure Energy makes the tank. The question I have is, I'm looking to buy some extra O rings for it but not sure which to get. I was thinking of buying some on Ebay, but I'm not sure if they'll be the same size, or if they'll work the way they should. Is there anything specific I should look for? Or are they all pretty much the same? Thanks for your help.