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

cabelas.com -- Another 25% of my ammo comes from here, just be ready to wait for the backorder

midwayusa.com -- outstanding company, I buy a lot of stuff from them

natchezss.com -- I'll buy from these guys when it's the best price

Those are the ones I have personal experience with. The ones below are on my list but I haven't been buying ammo from them.

aimsurplus.com

ammobank.com

ammunitiontogo.com

luckygunner.com

ammoseek.com

ableammo.com

atlantaarmsandammo.com

georgia-arms.com


I have no affiliation with any of these websites other than as a customer for the first four.