They are easy to make. What you need is a Co2 bottle which you can buy at most sporting goods/paintball supply shops.. a remote, which is a cord that runs from the bottle to the paintball gun. Get one with a regulator on it (a little knob that controls the gas flow that's on the end of the remote that attaches to the bottle).

Get a big container with a lid (I use a plastic jar that had pretzels in it). Drill a hole in the lid and run an aquarium airline through it. Hot glue it in the lid so there isn't any gap around it. Run the other end of the airline in a container filled with water. This is your exhaust, the water makes sure no air gets back in the container and revives the rodents!

Drill another hole in the side of the container, near the bottom and hot glue the remote end in there. That's where you pump the Co2 in. Attach the bottle to the other end of the remote, and you're set!

If that makes no sense, I don't blame you.