I have a drivers seat rip on the left side seat back and am looking to fix it.

It looks as if my cell phone holster and the seat don't get on well.

What's the best way to do this? New seat leather; a leather shop?

 

I cant really figure out exactly where your problem is based on the description you gave.
If it is a place that does not see a lot of stress then you could have it patched and redyed by a mobile interior guy. I just had my interior refreshed by them. It looks much better than before and cost about $250 for the entire cabin.

 

It's on the drivers seat, on the left side of the seat back; about where a belt phone holder would contact the seat.

It's about a 2" 3 corner rip.