You get him from Bill after passing through Goldenrod (past the Sudowoodo) and reaching the poke center in the next town (can't remember the name - haven't played it in a year). When you get there, Bill will come down, announce you can now trade to Red/Blue/Yellow, then bugger off back to Goldenrod. Go back to Goldenrod and find his house and Bill'll give you an Eevee for your trouble.
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