I used the "Jar Top" backpack made by Embark. These are available at Target and on their website. I picked up mine on sale for $20. They also have an "elite" version for $10 more with thicker/padded fabric. I opted for cheap and light. If I need to fix some stitching or add some duct tape later it will just make me like the bag better.

If you opt for a different bag you are really looking for two characteristics. The most obvious is that you want the storage compartment to be wide and deep enough to accommodate your discs. The other is that you want a bag that doesn't have zippers that split the bag down the sides. Remember that the whole point is to be able to walk around with it open and not be afraid that all your discs are going to fall out!