Just any random item...
Well firstly goto any pokemon editor and selection mega evolution to the pokemon you want to evolve.... then select the item you want it to held in paramater... then goto the pokemon that evolves after mega evolution (i.e if squirtle mega evolve into charmander goto charmander) then again select mega evolution and in paramater select first item i.e. ?????? And the pokemon it evolves to, select the first pokemon...
.
Make a script giving your player the keystone (currently yyy) and setvars ...(i firgot these) just check the game... in home town you will see the additional main character who doesnt say anything but gives you keystone... just check its script from a-map ... those steps are necessary... after doing so i hope it works.