What is a good source that will help me classify protists?
While not comprehensive, the Portal to Protistology has many resources for exploring the diversity of protozoa. I especially recommend their videos that are really fascinating, focus on distinguishing features of different protists, and may be helpful for identification to some degree. But given your interest you should just explore all the pages on the site, check out the books they recommend (e.g. the Illustrated Guide to Protozoa), and see what is useful for you!