For a search engine, I like wikiarms.com
SG, Target Sports USA, Academy, Midway ... all good outfits, IMO.
Of late, Academy has had some good deals (pretty much pre-scare prices) in common calibers -- .22lr, 9mm, 5.56, etc -- five box limit on any one item, $5 flat rate shipping on ammo.
Good spot to keep an eye on lately: mrgunsngear Facebook page. He puts up fresh online deals offered by various outfits, although you often have to be quick these days. The stuff is moving fast lately. Still some quite reasonable prices out there if you keep a sharp eye out.
Naturally, I pick up the odd box of this and that at the LGSs, too.