Academy Sports. Monarch brand (rebadged Barnaul) at $2.86 a box of 20. Thats like $143 per 1,000 for Barnaul. I clean them out every time I go.
Dicks also carries Wolf MilClassic for $2.99 a box of 20 and you get a 10% discount for ten boxes or more. If you buy a case of 50, you are out the door at $134.55 + tax. Sometimes they mess up and get black box instead, even better.
If you need to buy it cheap then the net is the way to go if you have no local outlet. Aim Surplus, SportsmansGuide, AmmunitiontoGo, Wideners, and JG Sales are a bunch of reputable places to buy from.
-Cabby