many universities offer nationwide registration with their research departments. i have had success with the ones below:
studies are pretty infrequent and just adequate paying (usually amazon). the most confusing part can be figuring out which studies are ONLINE and which are IN PERSON. once i registered for an in person study unknowingly. just really take the time to read the study--the word ONLINE will be somewhere in the description.
you also may have more studies if you proactively go the available studies site rather than just waiting for email invites.
here is an example of a current online study:
best of luck!