I get them on either Amazon or joann.com Joann usually has them on sale for 40-50% off. They are on sale now:
http://www.joann.com/search?q=projec...e%20protectors
Also Michaels and AC Moore both carry them and you can use a coupon. Certain designs from We R Memory keepers brand are also at Walmart, not sure about Target, but I generally order them from Joann.com or Amazon.