What profession/company is specialized in home air leaks?
What you need is called an Energy Audit. We had one done on our home several years ago. They go through your home looking for areas lacking in energy efficiency. One of the major parts of the audit is called a Blower Door Test. 

During this test they put a large fan into the door of your home and put your house under a vacuum. The result of this test - how much air the fan is able to suck into your home from leaks - determines how "tight" your home is. 

Our auditor went around the home with a smoke pen to identify areas where air was coming in - around switches, electrical receptacles, doors, windows, etc. Several days later a written report was provided to us which detailed specific areas to improve, ordered from most to least cost effective. 

The Canadian Natural Resources website has more information about these types of programs.