Compile a list of hospitals in your local area. Check their websites to see which ones do research in psychology and eliminate the others from your list. Contact these hospitals to ask them about psychological research participation opportunities. Sign up for a list in which they will contact you when opportunities arise. If possible, contact their research labs directly. If not, try getting contact information for the research labs by calling their general help lines.
Similarly, find a list of universities in your local area that conduct psychological research. You don't need to restrict the list to universities in your particular city, but keep it local because opportunities too different are not worth your time. Check their websites for paid participation opportunities and sign up for alerts if possible. If you can't find a website listing, contact each university's Department of Psychology to ask about their current research participation opportunities and the places where their researchers might post news.
Check your local newspapers regularly to see if any psychology research participation opportunities have been posted. Similarly, go to classified advertisement websites to check for the same opportunities. Be careful not to get scammed by fraudulent listings. If you are unsure, contact the organization that is supposedly running the study to confirm that the listing is legitimate. Avoid it if the organization is so obscure that you can find no trace of its existence.
Many organizations conduct their psychology research through the Internet, which gives you access to many more studies than otherwise possible. You can either start your own search by typing "online psychology research" into a search engine. Make sure that the website is legitimate because online scams are common. Search the name of the organization and the website to see if people are making complaints about them.