I know I'm 7 pages late, but the best way to find an Eevee is to use a Repel with a lv 18 'mon in the front. All you'll encounter are Eevee's.
Longest I've ever spent looking for something? Marill in Silver, for two days. Finding Marill's in this game makes me feel like I won the lottery. They're practically the new Ratatta. --- "Love, hope, faith. The greatest of these: love." Check out my webcomic! http://www.drunkduck.com/La_Vie_a_la_Mode