I bought mine at Wally World (Wal-Mart). My daughter bought her cute roller bag at her school. I have also seen cute and affordable one's at TJMaxx, but sale out fast. I know you will find something real cute on Ebay.
I bought a red rolling tool bag from Home Depot ! Worked great and was way cheaper than regular rolling school bags- plus it was a lot more sturdy and easily accommodated all of my thick / heavy books- and my husband gets it now that I'm done with school :-)
I bought mine at TJ Maxx for $20 and after having so much trouble with other bags, I LOVE it. It's big and open and rolls great and it hasn't even attempted to break on me. I paid more for bags at Walmart that annoyed me from the get-go.