Oh.... Please! I am a certified regulator tech for AUP products (Aeris, Hollis and Oceanic) and can tell you it isn’t rocket science although it does take some knowledge, skills and in some cases a few specialized tools. The OP wasn’t asking if they, should but which manufactures will support individual who wish to by supplying factory parts.
To my knowledge Edge, HOG and Dive Rite come to mind. With the Dive Rite brand a number of other manufacturers products are similar enough to use the same parts. I believe Zeagle and OMS fall under this umbrella but YMMV.
While on the subject of manufacturers marketing games... I’m a big supporter of MAP pricing vs. MARP and suggest all diver boycott brands with MARP pricing policies which price fix their products pricing instead of allowing the dealer to discount and negotiate their best deal. I’m starting to realize we should also be supporting brands that support end users with factory parts so we can maintain the gear we purchase.