You may be able to find a youth length stock or take it to a gunsmith and have him cut it to the proper length of pull to fit you then reattach the butt plate.
Numrich Arms lists replacement stocks but do not specify length of pull.
Numrich Gun Parts Corp. - The World's Largest Supplier of Firearms Parts and Accessories
Another source may be Brownells.
Brownells Search : Marlin 336 : Rifle Parts - World's Largest Supplier of Firearm Accessories, Gun Parts and Gunsmithing Tools - BROWNELLS