I have it, got it off linoone/elecrike/magnetric/wingull/zigzagoon. Not sure which, I was training my shiny ghasty I caught on the same day. PKRS is a big help, halves the time to EV train. and no, don't believe the noobies who told you it permanently halves speed. If it did, don't you think my aerodactyl would have 147 speed rather than 394?
EDIT: oops, forgot to answer the Q, the easiest way is trading with someone who has it already I guess, its random as you know, and I have played over 500 hours and this is the first time I saw a shiny + PKRS