That is a good question: All you have to do is go to Sunyshore City or whatever, and you go to the Market (grey building I think, looks sort of like a temple?). One of the people inside there will give you a riboon for your pokemon if its Ev's are all maxed out. Just make sure to have the pokemon as your 1st pokemon.
I have seen posts of people recieving the ribbon with only ~ 240 EV's
And I got a Bold Often Dozes Off one ready to give to you
I think I might be able to do this, actually, so I'll tell you if I end up making it work. I can get an Illumise, heart scale wish and then in double battle get smeargle to learn it and then breed it with eevee