Simplest answer I can give you is go here:
http://gun-deals.com/ammo.php
I don't know if they go looking for the deals or if the dealers notify them, but you're not likely to find a more comprehensive list of deals and it'll save you alot of time looking for them. You can have them sorted however you like just by clicking the label in the column header. I check them by cost per round (CPR column) and they include shipping cost in that figure. They'll let you know whos got the best price on what, you just have to go to the dealers site and order it then.