Oh boy... I'd stay far far away from Pet Smart. WAY too expensive!!
Ask around how local suppliers humanely euthanize their rodents. Most use a kind of gas chamber, from what I hear and see. I believe the other methods are becoming more and more unpopular, such as the disc (I think? or neck as you say) dislocation -- and no, putting them in the freezer alive is NOT one of those methods! That is totally inhumane.
There's always people out there with rats and mice
it's very surprising! If you are really having trouble finding a supplier, ask around for breeders. They will know a supplier, or breed mice/rats themselves and sometimes will have extra and sell them.
All the best!