Someone also mentioned the Lowepro bag finder. It may work in some cases, but when I entered what I wanted to put in the bag, it provided choices that would not work and omitted many that would. I found the Pro Runner 350 AW by looking through their site the old-fashioned way.
Even that's a challenge, sometimes. Camera bags are one area where going into a brick-and-mortar shop is a great idea. Bring your gear, put it in the bags, see what fits and what doesn't. Try the loaded bag on for comfort.
Online descriptions can be misleading. For example, someone above mentioned the Lowepro Primus AW - I bought one, used it once, then sold it. The desciption includes the statements, " It is made to carry a pro digital SLR or DV camcorder outfit with accessories, and personal outdoor items,
" and "Side zippered pocket allows for fast access to camera and lenses.
" In Lowepro parlance, 'pro digitial SLR' usually means a 1-series or gripped body. What the description doesn't state is that a gripped body cannot possibly be removed or inserted via that quick-access side pocket, and even with the grip removed from my 7D, it could be pulled out ok but was very hard to put back in without removing the pack and opening the main zipper.