When I was looking for a narrow sconce, the narrowest I found--including the backplate measurement--were on Restoration Hardware. Out of my budget and I ultimately went with just a fixture over the mirror (in addition to some recessed cans) because that worked better with my dimensions and budget.

My go-to online store for things like this is build.com. Here are the narrow sconces I recently bought from them for my bathroom: Kovacs sconce

Just make sure before you fall in love with any sconce that it is rated for a bathroom (i.e., "suitable for damp locations"), and that it has enough wattage/lumens for the room. Sometimes manufacturers have several different versions of the same sconce but the lighting source used (halogen, LED, fluorescent, etc.) will put out different amounts of light.

I think we all know that sconces are supposed to be best, but I have the dreaded over-mirror light, and they aren't a terrible problem. In the spirit of addressing every option, here are a couple other thoughts:

- Hanging pendants

- A single sconce on the left-hand wall partnered with an above-mirror light.

- Add a small wall between the vanity and the toilet so you can have "side wall" lights on both sides.

- An above-mirror light coupled with in-mirror lights.

To add to mrspete's thoughts, I went with a pair of sconces on either side of the mirror, and the sconces I picked out had a matching overhead fixture which I figured I'd buy if the sconces didn't provide enough lighting. As it turned out, the sconces were entirely adequate, but it's something to keep in mind.

Can your medicine cabinet be recessed at all? If you are down to the studs, now is the time to be framing out the recess for the cabinet, cut the studs box it out with header.

You say the vanity has a side wall on the left side. Open up that wall, create more storage there with an additional, preferably recessed, medicine cabinet.

Think about the projection of the lighting fixture, particularly if you go with over the mirror mount. You want to try to get the light coming out beyond the projection of the cabinet, this avoid the cabinet creating a shadow. Projection is a factor, as is the distance above the mirror you can install, the shape of the light, the directionality of the light, etc.

With a 48" vanity I think a 30 - 36" mirror with a sconce on both sides and a second side wall cabinet would be great.