You can go to the Castelie sewers and eventually after you go up some stairs, you can get to castelia gardens. There is just one tree in the middle with tall grass soronding the tree. It is hard to get and if you can't get to castelia sewers go to youtube and search.
If you want a garenteed chance to get one just go to castelia city and go to the third floor where the medal box place is and go to the third floor (fennel's lab). Then, fennels assistant will give you an eevee (have an open slot in your party) if you talk to her.