I have a large oak tree in my yard that was struck by lightning several years ago. The strike left a large scar (about 7 inches wide and maybe 5 inches deep) of dead wood extends from the ground to at least the top of the main trunk (about 20 feet). I would guess the tree to be between 4 and 5 feet in diameter near the bottom of the main trunk and about 100+ feet tall. This deadwood is infested with a small flying, wood boring insect as high as I can see. I'm concerned that I may eventually lose the tree because of it.

I have called several exterminators and they all said they don't do tree work. I looked for other types of experts in the phone book that may do it but to no avail.

First question - What type of professional should I be looking for to get rid of the bugs.

Second question - Are there any do-it-yourself insecticides I can use to get rid of them and what specifically can I use?

I would call a local plant nursery and ask them. If anyone in the area would know who to contact, they should.
In fact, call 2 or 3.

You need an arborist............and he may be at a tree triming business............