I get my rodents from American Rodent Supply (www.americanrodent.com). Their stock is awesome and the people are very helpful.

I believe frozen rodents can be kept up to six months in a plastic, tupperware-like container in the freezer.

I usually order one size of rats for all my snakes. If they seem to be still hungry after taking one, I just give them another. Then, when you run out, you can move up a size. But that's me.