Hi 5 thumbs,

you can get shims (0.25, 0.5 or 1.0 degrees) from any of the guitar building stores or ebay or similar. Most seem to come as a set of 3.
Or Simon B put a post up about making your own shims from scratch (I cannot find the post!).

I would assume with a wooden shim it just gets glued into place when the neck is being glued in.