Grove Art Online contains scholarly essays on many approaches to art history which are often an excellent introduction to the subject. Also from see the Concise Oxford Dictionary of Art Terms and the Encyclopedia of Aesthetics: a search in either will return results from both.
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:
You may also want to run supplementary searches in these databases: