That's a neat idea. A Bake Sale might work too, though you'd have to control yourself around the inventory.
I save up Cathe-money a couple of ways.
1) My Cash-Back money earned using my credit card (The balance is paid in full each month though, otherwise you aren't really saving money by carrying a balance.)
2) Holding back on purchases I would normally make and instead redirecting the money to whatever special item I'd like to buy. It could be money spent eating out each week, or the amount spent buying coffee every morning, holding back on buying treats at the grocery store, etc.