I just ordered a can (420 rounds) of the Federal (Lake City) .62gr green tip stuff from Brownell's tonight. This is the same ammo used by our military. Paid $149.95, plus $12.95 shipping (including a shell feeder plate for my LnL case feeder, and a set of Brownell's screwdrivers...).
I thought this was a pretty good deal for the good stuff. LGS has the same can for $179.95. I think Midway has it as well, on sale for the $150 price, but I have been staying away from them lately because I'm tired of them bumping the shipping charges up to cover more profit for them. The .62gr is a little more pricey than the .55gr Federal ammo, but this is my Zombie defense stash; steel rod core...! I also have a stash of .55gr for general plinking. I don't/won't run steel case ammo in my ARs.
I agree that the IMI ammo is good ammo. I got a bunch a few years ago for $4.50/box of 20 (.55gr fmj). Bought that at LGS. I should have bought all they had, it is very good quality, and shoots great.
Natchez usually has good prices on 5.56/.223 ammo. I was going to order from them this time, but they did not have the other items I needed... I've also had good luck with Wideners and Powder Valley.