Try vinyl sideboard from hardware store for all kinds of shimming; it is very easy to cut into the shape you need, and then I just fix it in place using ductape. If you want thinner shims, use trail maps.
If that doesn't work, explore your boot for bells and whistles that accomplish essentially the same things as you would with sideboard, but more precisely.
In fact, the only bootfitting part that I bought at a ski store was the Superfeet footbeds. It's a lot cheaper that way, but you get what you pay for ...
Oh, and instead of cardboard from Coors 12-pack, I remember using thick paper plates 2 pairs of boots ago - they provide a lot more cushioning.[ March 18, 2003, 11:48 AM: Message edited by: AlexG ]