[T. O'Heir]

You have to try a box of as many brands as you can to find the ammo your rifle shoots best. The price of said ammo means nothing.
If your local gun shop is telling you he can't get multiple brands it usually means he doesn't get terms from his suppliers. There should be no reason for him to not be able to get you whatever you want. Especially when you pay up front. That's a normal thing when ordering anything. Order a box of as many brands as possible. Six boxes isn't enough to make it worthwhile though. Six to 10 bricks will be.
Talk to your shooting buddies about a group order. Ten shooters buying 10 bricks(500 rounds per brick. 10 50 round boxes per brick) of different brands means each one gets a brick of assorted brand ammo.
Failing that, is mail ordering an option? Shipping will hurt(not so much if 10 guys are helping), but you can mail order just about anything.