Hi Michelle,
I don't know where to get the cheapest, but there is a pampers and also a huggies diaper website that both have mailing lists that you can sign up for, they will send you good coupons off on their diapers through the mail. Does that help any? I know... so many diapers, right? I get them for our local human services through these sites and they either give them out or get diapers to help people out with them.
Another place you can try... do you have a Human Services department in your town? We do, but it's a small town... and they usually have diapers that they'll help you out with, in that case, they'd be free, and ours is in our Town Hall. Give it a try?
Good luck, let me know how you're doing and maybe I can find some other sites or something!
Kat
