The 50-65 shim gives the most drop when the 65 is on top. However, keep in mind that you're not just lowering the comb, you're increasing the drop of the entire stock, changing the angle of the stock to the barrel, and changing the pitch. If just a lower comb is what you need, you need an adjustable comb.
There are a lot of gunsmiths that can do that, but TRON on this forum is the man.
Shims are available from Coles Gunsmithing. Google it. Also I believe Brownells has them. You can't get them from Beretta.