[scoobydrvr]

Just my $0.02:
I looked into having these made here in town and the biggest thing I ran into was the difficulty involved with milling the phenolic material. Most traditional CNC machines won't tolerate much work on the stuff because it is so abrasive. It'll wear out your tool very quickly. The other problem is that the dust is carcinogenic! The one place I can source the material locally has to shut down their shop, clean everything, cut the material, and clean everything again before they can resume the work.
I'm not sure what kind of a CNC setup you have but I hope it can tolerate this kind of material. A water jet will be your best bet if you have access.
Additionally, the G10 GRE that Svx95 mentioned also goes by the name of "FR-4" and "FR-5". They aren't quite the same stuff but almost identical in terms of performance to G10 (the "FR" stands for Fire Retardant). FR-5 has a higher temperature tolerance than FR-4 but both will work for normal applications. You might be able to keep costs down by using FR-4 instead of FR-5 for stock applications; my local supplier wanted substantially more for 5 than 4.
Do you need CAD files of the gasket cutouts or have you already made those?