pre-killed mice...

Umm, you don't put the mice in WITH the dry ice. The mice are in another container into which the CO2 seeps. :)

Another way of doing it is to use CO2 cartridges used for paintball. They're refillable and don't cost a ton unless you get the huge ones. :)

I was going to mention a paintball cannister for C02 but Argentra beat me to it. There isn't an effective way for most of us to store dry ice, so if you need to pre-kill a mouse every week you'll need a fresh supply of ice just as frequently. The paintball cannister is something you can always have on hand.