Clinicaltrials.gov has all clinical studies, if they’re recruiting, and can enroll you in a study.
https://recovercovid.org/ is the national program led by the NIH that I’m in.
I’m in Chicago so I looked at the top university hospitals. Northwestern had a Covid program I joined in May 2022. I’ve had every test possible done on me, even tests not fda approved.

I didn’t even knew there was such a thing. It was early days of Covid and I just called the major Chicago hospitals and medical universities to see if they had anything. They’ll pay me $20 to come and and get very sophisticated tests, some being approved by the FDA by testing on me and other volunteers. Works for me