Use Cheaperthandirt as a last resort, Midway or Wideners is much better, or Ammoman. Myself I trust Cabelas, they have never charged me first on backorders. I hear Midway is also good, some outfits will charge you first and then you will be waiting for weeks and weeks.
I have never used Cheaperthandirt because their prices always look high. I use Cabelas all the time for run of the mill target ammo and ammunitiontogo for hard to find or novelty type stuff (tracers etc.). They also have a lot of other stuff besides ammo. I just picked up a bunch of surplus 30 cal. ammo cans for $2/ea. I see these all the time at gun shows selling for $12-15.