for storage, try to stay below -20 degrees celsius or -4 degrees fahrenheit. if you are just a bit below freezing, worst case is that bacteria in the guts of the mice or rats get active, and produce enough heat to keep the belly of the mice/rats above freezing temperature for a while. Then they basically internally rot in the freezer. Freezers of the crate design, a box with a lid on top, tend to be most energy efficient for this.
Are there any reptile shows coming to your area? I think that would be your best bet to get a good number of decently priced feeders. When I used to bulk order frozen, I would rebag everything in to Ziploc sandwich bags. It's a little time consuming, but it prevents freezer burn pretty well.