I would get my recommendation from my GP.
The only caveat to that would be if I was working with a Consultant, or a trusted friend or family was.
Firstly, depending on the problem, you need to decide wether you need seeing by a nephrologist or urologist, they each do different things with the kidney. GP!
Just a thought but it may help if you state which region you're in. If you live in Glasgow and somebody on the forum recommends a doctor in London are you going to travel?