Rogers has decent prices and free shipping at $99. Sign up for their emails and get first notice of their flash sales. I cleaned up on some 9mm +p jhp ammo a few months ago. It was less than half price of other online places and in stores.
I've ordered from freedom munitions, sgammo, and capital cartridge. They've tended to be my best ammo buys, although I used to live in Oklahoma so sgammo would have me ammo in a day or two with almost no shipping.
Some good offerings above -- Also wikiarms.com, which has a weird relationship with gun.deals but all good info for keeping up with prices. What's hard to pick out sometimes is shipping costs vs price per round in the listings.
As others have said, you'll want to constantly check ammoseek. Shaw Arms has been killing it when it comes to 9mm lately (free ship with code and no tax).