MLS availability varies from region to region according to the local board of realtors. And, even in regions where the MLS is generally available, that availability may be limited to specific agencies, with others not participating at all.

(Once upon a time I did some I.T. and software maintenance work for a number of local realtors and the local board.)

Oh, and also, various MLS interfaces (e.g. Paragon) provide a lot more information on properties than non-realtors have access to.