Oxford Art Online contains scholarly essays on many approaches to art history which are often an excellent introduction to the subject. There will often be more than one essay, because this resource began life as several independent reference works: it can be worthwhile reading them all.
Summon of course finds much relevant material. Searching for methodological topics is not always as simple as for more concrete subjects. It is often necessary to experiment with different keywords and different ways of structuring your search. See the quick guide to Summon advanced search, which is part of the Summon guides series, for tips. Here is an example of a search which works well for finding material about feminist approaches to impressionist art:
Art & Architecture Source is an important database of journal articles about art, only partly included in Summon. So it's worth searching separately.