Buying bulk ammo is always a bit of a moving target since various vendors have differenct prices and inventories at different times.
I don't think anyone has mention it yet, but I've had good luck with ammoman.com
I know this will probably get me flamed, but I've shot thousands of rounds of Wolf 5.56 through the various uppers I have and have never had any trouble. So don't overlook them despite what people on the internet may say.
On the other hand I've had nothing but trouble from Tula rifle ammo.