Thicker gaskets with a coat of Perma-Tex High Heat spray a gasket added to them for insurance.
Check out a couple of upcoming swap meets in your area and see what's around. E-Bay but be aware that they are a Buyer beware situation.
If considering the self-machining route, Might be possible to do it with a rigid sanding block like body-men use to block sand panels. Use 400 or finer Wet/Dry paper or emery cloth and wet it with machinists tapping or cutting fluid. I have seen a block decked that way but it was more of a cleanup of the head gasket surface rather than an exercise to remove material. Would say this is a last resort option.
If the manifold has some history with you then the machine shop. Question? Does your local college have a machinist's training course and do they accept walk in customers with work to be done that a senior student could do?? Know anyone who happens to have a milling machine and would trade work for booz, work, parts, ???
Nick