I have looked on <clinicaltrials.gov> but found it mind boggling in its length and detail. You look under your city and your disease.
I have tried out for several trials but only been accepted to four. My teenaged children were in a couple. At the time, pharmaceutical companies wanted to know how meds were tolerated by younger people and women, so they were actively looking for those populations for their studies. By being in a clinical trial, I found out I had a disease I would probably not have discovered for quite a while.
Another way to look is to go read the bulletin boards at a research hospital or a medical school. Some have offices where you can call. I also looked under "Research" in the Yellow Pages. I now have about a dozen of the research centers' phone numbers listed in my phone. They say to call every few weeks, but, most of the time, I just wait until they email or write to me because I am so busy. Sometimes they'll call me to ask if I am on the same medications because I might qualify.
I think the pay is usually about $35 or $75 per visit, or $200 a study of a few weeks, or something like that. I think one paid mileage from outside the city. I have been given equipment such as blood pressure monitors and a device that measures whether you can take a good breath when you are having an allergy problem. The medical care is related to what they are studying, and, if the doctor finds something else, you will be referred back to your regular doctor to be sent to a specialist. The participant also learns more about medical procedures and new evaluations that have come out, especially if she/he asks questions and pays attention to what the nurses are doing.
I have a neighbor who participated in a week-long sleep-in study for male diabetics with depression and sleeplessness. He was paid $1,500. One of the local research companies specializes in stays of that length. When a person stays, their exercise, diet and meds can be more strictly controlled. When I tried to get in one one time, they told me that they would even do searches for hidden food. It's a very nice building with pretty landscaping in front. I could imagine finishing a good book during that stay! I never was accepted.
There is a study that the National Institutes of Health and the Centers for Disease Control do every year or so starting in about February where they get a consensus of what the American public's health is like. It is the same study that tells doctors whether your baby is growing at the correct rate. I went for the portion that tests their computers to see if they were recording the information correctly. They looked in my mouth, measured my back, etc. I was paid $100 for about an hour of that. To this day, I still don't know how the company that called me got my name, but they recently referred me to another study. Unfortunately, I didn't fit into the age range.