See if you can locate a local reptile breeder near you that would be willing to sell you a half-dozen or so mice for the time being. Most will have a sulplus on hand that they will be willing to sell. What area do you live in and what is your email address, perhaps someone here could contact you if they are close by.
Also, consider that as a permanant solution too. Buying mice off of a local breeder, while not as cheap as buying online in 1000 lots, is still probably alot cheaper than what you'd get at a petstore. I do that, and my food bill is about 1/4 of what it would cost at a petstore -- Just a thought.