Levelling bookcases

[Jim Franklin <jamesf@groupwbench.org>]

Here's one for a quiet Saturday. I'm installing a couple of walls'  
worth of Ikea bookcases. They will sit on an 80 year old hardwood  
floor which is no longer close to level. I need something that will  
"disappear" under the doubled up 1x supports of the bookcases.  
Softwood shims are too soft and are wider than what they need to  
support. Any idea what would work here? Thin hardwood shims would be  
ideal but I've never seen them.

thanks,
jim