You could try www.themousefactory.com and check what their minimum order is as far as number per bag and shipping costs. They have an 800 number and Edna is very helpful and informative about her product.
Just a suggestion though, before ordering frozen. If you have access to a local supply through say a decent pet store, you might want to let the snake settle in, feed it for a few weeks to make sure what size is right and that the snake will eat it readily for you. Wouldn't want to see you stuck with a bag of stuff the snake is refusing to touch. From what I understand, fully frozen mice/rats are good for at least 6 months but do check with the supplier about that (I feed live so not sure on that myself).
There's a lot of great information here as far as settling in your new snake properly so that you up the odds of a healthy, stress free snake that has a great, consistent feeding response. Just have a wander through the forum....gotta love that search button LOL.
~~Joanna~~