I just use a piece of .010 plastic binding that I lay flat in the back of the neck pocket. You can also use wood but it can compress over time unless you use a hardwood like maple or ebony.
Most people use a piece of cardboard from a cereal box or business card stock. If you want accurately cut tapered shims because you believe it will impact your tone 
or just want a good shim, check out Stewart MacDonald.
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