Stick to Elgyems.
Elgyems give out 1 EV in Special Attack as well, and are hit super effectively by your Bug type moves. And, they are found at the same place and level, so it won't be a problem battling them.
If already aren't holding a Power item to increase the EV's gained, you can give your Joltik a Smoke Ball (Found in Castelia City, I think) to hold, so that it can escape from all the Litwiks without fail.
At that level (or any other), I am not too sure there are good Special Attack EV Training spots.
That said, I just wanted to also add: If it is for ingame purposes only, then EV training doesn't change things by that much. You can still do it if you want that tiny extra punch, though.