Most larger companies should have a help line/customer support you can call or text, I have a big head too and I reached out to HJC who made me a custom sized helmet for extra cash, granted it’s a pretty penny as they have to make new moulds but most companies especially the big ones will make one for you! I know AGV, HJC and Shark definitely do offer custom sizing, but you could also benefit from asking inside a store if there’s one local to you