I also got an ATG dispenser for Christmas & have been shopping around for the best deal on refills. Framingsupplies.com has 24 rolls for $3.22 per roll plus shipping.
According to the 3M website you can purchase refills at Joann's and the currently have a 50% off coupon. It appears Michaels has the best deal when you use their 40% off coupon (no shipping required). Let me know if there is something better out there?