http://www.gunbot.net/
I just stumbled on to this site that monitors popular caliber ammo availability on the web. It has options for sorting by price per round and in stock only listings. they monitor the following sites
ABLEAMMO
AIMSURPLUS
AMMOBANK
AMMOMAN
AMMOTOGO
AMMUNITIONDEPOT
BROWNELLS
CABELAS
FREEDOMMUNITIONS
GRAF
JGSALES
LAXAMMO
LGOUTDOORS
LUCKYGUNNER
KEEPSHOOTING
MIDSOUTHSHOOTERS
MIDWAYUSA
NATCHEZ
PALMETTO
PRIMARYARMS
SGAMMO
SPORTSMANSGUIDE
SURPLUSAMMO
TARGETSPORTSUSA
USARMORMENT
WHOLESALEHUNTER
WIDENERS
BCM
44MAG
EAGLEARMORY
RAINIER ARMS
RIFLEGEAR