You can buy a backpack from a cycle shop - but, they're all the same for the most part - except for the price. Get one from a sporting goods store that has a selection that has reasonable water repelling characteristics, has many pockets and compartments, and largest interior size. If I was buying a replacement right now, I'd lean toward one that has a removable waste strap (less flopping). To give you some idea of what I mean by they're all the same - I've used the same bag for about 10 years (10 years of back and forth to work in the car 5 days a week), 6 years of medium distance bike rides, several camping and fishing trips, and bunches of vacations. It shows little or no wear and it cost me about $20 and gets thrown about all the time. It's a regular old Eastpac if that makes any difference.
