There isn't a hazmat charge on ammo no matter where you buy it. You only pay the hazmat fee if you are buying primers or powder separately. If the ammo is already loaded they can just ship it as ORM-D.

Natchez.com, Ammunitiontogo.com, PoliceHQ.com, are all good sites to check out. There are a ton more I'm sure, but those are the ones I know off of the top of my head.