1. Visit the website of the museums you visited or where the art is located. See if anything is written about your piece.
2. Look around their site for more description about the style or time period in which your piece(s) were made.
3. Visit other museum or gallery sites (Google search artist name + "site:.org" or "site:.edu" for most museums, or just the word museum) where your artist has work displayed. Repeat step 2.
Yes, read Wikipedia and whatever else looks academically appropriate to get some basic knowledge on the place and time period in which the work was created. You do not need to cite things that are considered common knowledge. Basic Googling is a good first place to start gathering important key words to use to search later.
Art research, including art history, music and literary criticism, is a little different than science or current event-related searching.
Current? For art, this criterion is less important.
Relevant? Reliable? For art history, your job is to tie together historical, biographical, artistic, ideas and individual interpretations can be valid.
Authoritative? This criterion matters in art history! Living artists, scholars, and museum professionals are authoritative sources.
Biased? Everyone has an opinion about art, but we want to quote people who can really talk about it critically.
Is this fact or opinion? Is it biased? Is the creator/author trying to sell you something?