Either out of your own pocket, or on the class/subject budget? Until today, not that I've been going at it terribly long, it would have been a pack of A3 playing cards. Fairly pedestrian. Just now, though, I bought myself a job lot of old mortice keys off eBay for use in a "creative resource" assignment for University. The idea is to build a physical link between them and the "Thinking Keys", and get the children in the class to do some self-directed work using them.