I sometimes buy a pre-packaged one at Wal-Mart for under three dollars, which is a good deal.
The wife buys this plastic tub deal that has some really good lettuce mix in it from Wally World for cheap. But this thing my friend gave me is like 6 times bigger. It barely fits on half of our refrige shelf which is one of those huge double door deals that you can stack two cases of beer on a single shelf and still have room left over...