Call around to mills and feed stores. Ask if they carry purina products, and ask if they can give you a quote on it.

Example:
Hello,

Would you be able to get a product called Mazuri mouse breeder 9F? It's a Purina product. And if so, what would be the price per 50# bag? Is there any bulk order discount? Any help you can give would be greatly appreciated.

Mazuri Mouse Breeder 9F 5M68
Product Form 3/8" x 5/8" Oval Pellet, 1" long
Catalog #0006758


Thanks and best regards,
Okay, that was more email form, but a lot of feed stores will look it up and give you a price if they carry Purina.

Yes, they likely have to order and it can take up to 3 weeks to get it in. So, buy in bulk and order before you run out.