I just saw the write up last night for doing your own CO2 chamber with dry ice and a cooler and some paper towels. It will probably cost you $30 at first for every thing, but once you do it, it will probably be cheap as hell.

Put the dry ice in a container that has holes in it. Place the rats/mice into the cooler. (yes, like a snack drink cooler). Make sure you have paper towels down. Once the rats/mice are in, place the container with the dry ice in it, into the cooler and close the lid. Within 5 mins you'll have a bunch of rats/mice that went to sleep, and never knew what happened.

Then you can freeze them if needed. That way it's humane and you don't have to violently kill them.