bought my pair at a LBS, but they're too heavy (12 lbs!), so i took 'em off... i then made a pair out of a set of soft-sider igloo bags (about $6 at the local Big Lots)... nice for hot weather, but not as secure with a heavier load..

but honestly, i do my weekly Farmer's Market shopping using just a cheap Nashbar messenger bag and haven't run into trouble...

of course, i'm only shopping for two, not a big family...

other folks have used the big kitty litter buckets to good effect (e.g., ScoopAway):