Unless you're extremely lucky not likely to find the bag second hand on the cheap. Sears wants $52 and a guy on Ebay wants $40. Sears/Roper/AYP had literally hundreds of variations. Plus the bag is a wear item, so there are more mowers than bags out in the universe.

On the other hand, if you start calling around to mower shops, lightning might strike. Even a blind squirrel eventually finds a nut.

Just buy a new bag from Sears rather than trying to save what will be $20.00 or so. It should last for years.

Assuming you already have a bagging kit but just need the bag only, it shouldn't be too hard to get.