It's actually a military surplus bag I picked up for $10. Waxed canvas with a water resistant liner, too. It's up for debate what the bag was used for, but the guy that sold it to me said it was an ammo bag. It holds about 1 grocery bag worth of stuff, so its pretty roomy.

All I had to do to adapt it for the bike was to cut out some of the stitching on the wide "belt" that goes around the outside to slip the seatpost through, and then use key rings to attach the upper eyeletted tabs to the seat tabs. Took about 15 minutes to modify, and works great. So far I've had about 20 pounds of gear with no problem. The straps are long enough that you can lash down a bed roll, tent or sleeping bag, etc.