I haven't seen a cost effective replacement part for bespoke bladders (any with the welded/glued in flange), but I have made washers for someone to deal with the larger hole size. PM me if your interested.
I believe Happy was referring to buying actual SP parts, but there marketing methods in the US make it extra difficult for us to get here.
I ended up replacing all of them with the DGX flange kits. Not sure how I feel about the repair.... The flanges only give you about 0.125" of sealing surface if the seal and flange stays perfectly centered which is hard to keep centered once you start tightening. I probably need to mark the flange outlines once centered and re-thread to ensure they stay centered while tightening. That said it seems to be holding air with me eye balling center and tightening. I did a lot of searching for SP repair kits and could not find repair parts here in the US. As you stated they don't appear to want customers to repair. It seems like I need to get Steve Wozniak on the case....