The easiest way to obtain this item is by checking all the bards using the pickpocket skill or a scroll of oracle. Every time a bard gets a new instrument it has a chance of being Stradivarius. Thus, if none of them have Stradivarius in their possession, be sure to steal or destroy (through liberal use of fire damage) their old instruments, before killing them, and they will get a new instrument once they respawn. It can be purchased with Stoke with 9000 music tickets from Elona+ version 1.66.