i'm not sure you will find any company that specializes in this but pretty much any decent carpenter could do it for you. i have 4 great danes and had mine installed in the wall.

I would start at Angie's List and look for a local handyman. Look for one that does carpentry. If you have a local hardware store (not big box) ask them if they know anyone that could do the job.

It's not difficult but one needs to understand the type/style wood door you have and placement of the pet door.