For online purchasing, here's where I go:
ammoman.com -- I buy at least 40% of my ammo from these guys, make sure to fax them a copy of your Drivers License when you give them your first order
A few years ago at AR15.com there was a big deal about ammoman (Eric, IIRC) selling an SKS as a Vietnam bring-back. It wasn't; he seriously screwed the seller and his reputation in the whole deal. There are reports of messing up orders, blaming the customer, and not making it right.
I have ordered from him years ago, but now can find most things cheaper elsewhere with little effort. I won't buy from him anymore.
YMMV.
Caveat emptor.