People usually follow the cold emailing approach, which means searching for the big institute like Mayo, Harford, Cleveland, etc. and trying to send an email to the lab director. Choosing the right timing after the Match season ends when a lot of research volunteers have been done, leaving their spots, and hoping someone replies to your email, this approach helped different people across the world find unpaid spots. Another thing is that if you have connections with someone who works at a big institute where there are big research labs, they can help forward your application to lab directors.