Since many folks have mentioned medical tourism, I'll mention three other options.
1) Forward Medical - I have had nothing but good experiences with them but they are not going to solve, just help diagnose & are a replacement to primary care.
2) Working from your symptoms to try to find a community online that has built up around this. There are several, I know that I listened to a notable podcast recently about the person figuring out their diagnosis related to lyme disease when doctors couldn't figure it out. Once you figure out a pathway, then you find a specialist and test yourself. Doctors are not great at looking outside the box for stuff, so if you have something uncommon it's going to be hard.
3) Related to 2, once you have an idea, connect with some top doctors, especially researchers at top tier institutions (think Stanford, Zuckerberg hospital, Mayo, top clinics (one in Berlin I can't remember), etc. Maybe ask a friendly doc to also post on Sermo for you (a medical focused social network) to get a broader audience. I just read the new Michael Lewis book about the pandemic and there's this guy Joe DeRisi at UCSF whose lab I guess basically looks at absolutely crazy things and fields requests all the time, might be worth calling although it's a long shot.