I recommend using indeed.com and searching "psychology research assistant" or "psychology research jobs". You can limit the search to different geographic areas. Also, studentdoctor.net (Psychology PhD forum) usually has a thread where people post positions that are available. April/May tends to be the time when most positions are available as currents RAs find out about their graduate school prospects and employers start looking to fill newly available positions. If you're currently working with a research mentor, some of their colleagues may email them with requests for applications. Maybe you can tell them you're on the lookout for a position and to keep their eyes open? There are also email lists that employers tend to use to blast out calls for applications - this differs according to your area of research interest. Good luck!