Eevee can learn it by TM, so that's the easiest way to go. It's 64 BP at Battle Tower.
Otherwise, Eevee is in the Ground egg group, which is one of the largest. The long list of pokemon is here:
See if your pokemon that knows Shadow Ball is in there, or perhaps can breed with one of those. Smeargle is in this group, so you could Sketch the move onto him (from a double battle with Smeargle and your Shadow Ball pokemon) then breed for an Eevee.