A useful site to consult before making a trip is the “Red de Museos para la Atención a Personas con Discapacidad” (Web of Museums catering to handicapped people)… while not infallible, it basically lists what services are available to those of us who are differently abled. The site is, of course, in Spanish, but easy enough to decipher. Most major sites (including the National Palace and the Antiguo Colegio de San Ildefonso (site of the famous Rivera and Orozco murals) do the best they can, but those painting run up staircases that make it difficult to explore in any depth. The guards and staff in museums here, will do their best to cater to interested visitors.
Some museums allow wheelchair users free access. A few charge more admission for foreigners than for residents. Sundays are generally free to all.