I went to a Grand & Toy (big chain of stationery supply stores) here in Calgary and asked about the credit card sized fresnel lens. They didn't have any in stock but they ordered a couple in for me - $0.99 each (Canadian). (I ordered two because I felt guilty about making a special order for less than a dollar.) <img src="/images/graemlins/blush.gif" alt="" />

Many bookstores will sell a fresnel lens bookmark which will work at least as well, but may cost a bit more. (Otoh, you may be able to cut it in two and get two lenses out of it.)