Not sure where you should look, but you can get Teflon guide cutters and do it yourself.
They're self-limiting and will stop when you get to the right place.

Only other thing you need is a 1/2" drill motor.

I did my own on the 462" Buick engine, didn't take long and no big deal.


You may want to check with an auto machine shop as well.
They don't seem to charge much to cut the head for Teflon seals when they do the valves & seats and perhaps that would be true when cutting for seals only.

The price may be favorable enough that you could have it done and skip buying what could be an expensive tool.