I wouldn't suggest Comcast or an electrician. Most electricians (there are some exceptions) don't have the knowledge to properly run and terminate ethernet cabling. With Comcast, you might get lucky and get a skilled technician but they are few and far between.

Look for a company that installs home A/V equipment or computer networks. Companies that configure computer networks don't always install cabling, but they could refer you to someone.

I'm including a link for the CEDIA trade association dealer locator, it would be a good start.

Here is a link that might be useful: CEDIA - Find a dealer

Retrofitting Ethernet cabling will be a major job and will typically be very costly (labour). A lot of planning will have to be done to minimize cable length, number of switches, ...

All the cable will have to be run I a star pattern from the main router location and avoid close proximity with power lines.

A wiring closet will be required along with Ethernet switches.