You may have posted this similar question some time ago as User ID "B" - if so, then this is a repeat answer for you. If not, here were my thoughts:
Most flooring contractors will do this, as can carpenters. Search the List. I can't imagine that anyone "specializes" in squeeking floors and stairs - just an incidental job for various types of contractors.
Important to REQUIRE them to trace out any electric or piping in the flooring first, because the screws needed to hold the plywood or particle board sheathing down may be long enough to penetrate pipes or wires run through the middle of the joists.
Obviously, best time to do this is when replacing flooring, because then you can see all the nailing lines in the sheathing, plus you are not going through existing flooring (or having to work from below) to get at the problem.