Buying rodents live from a pet store is expensive and they are always really small.

Have you considered buying f/t from a company an having them shipped to you? You would need freezer space for them, but it is a much more economical way than spending money on questionable mice from pet stores.

I would personally recommend The Mouse Factory (http://www.themousefactory.com/index.html). They sell both mice and rats turn out a great product

As for Robin's worry about a vet visit: It is a real consideration. I would encourage you to sock away money each week, if you can, so that you have $100 (minimum) saved up for an emergency.