I don't buy much ammo online, mostly just components.
Check out the forum sponsors; you'll find reasonable prices at most. Even if they are a buck or two more in some cases; I'm more than happy to spend it with them. Sam doesn't let just anyone advertise/sponsor on the forum; all those I've dealt with have been great thus far.
In today's environment, the best place is the site that actually HAS some in stock!
I don't purchase loaded ammo-roll my own but have friends who wanted to get large (1k) amount of several different calibers so I went on a search.
For the common stuff like 9mm or 308 it was tough-finally got it from bulkammo.com and precision delta.com decent price fast delivery both guys were happy with what I found for them-local gun shops either had nothing or 3x the price what they paid online.
Have also sent friends to ammoman.com with good results.
Hope it helps a little.