Writing Literature Reviews by Working through this checklist at least two times should identify the vast majority of the literature relevant to a research question. Begin with what you know. By the time you finish the first pass, you should have acquired additional terminology that will make the second pass productive.
After your first passes at collecting sources in the literature, use the sources that are the most relevant to do some citation chasing. Both psycINFO and Google Scholar list what other research has cited a publication. Remember to chase citations both ways.
Literature from other disciplines: using OneSearch, we can discover a large amount of relevant literature from disciplines other than art therapy and psychology. Unlike Google Scholar, you can use limiters in OneSearch to focus on discipline specific databases. On the other hand, there is an argument to be made for using those specific databases (PubMed, for example) in the first place.
You may also wish to review the library guides (libguides) belonging to other, relevant disciplines. This will allow you to identify discipline-specific databases that you can then incorporate into second and third iterations of your lit review process.