Depending upon your budget and your requirements for size, think surplus.
Several places sell these as "German Special Forces Backpack." I dunno. Here's a pic:
http://www.usfreeads.com/1814780-cls.html
I bought one of these for hunting and I found that there is a space between the side pocket and the main compartments. I could slide my mauser carbine into that when I was riding my mountain bike into/out of the woods.