Glad you like our site!
We occasionally list art events and openings on our events page, but Tokyo Art Beat is probably the best source of art events in Tokyo - here's their openings page:
For more contemporary and underground, the agency TokyoDex organise excellent events: https://www.facebook.com/TokyoDex/
Also Facebook is a popular platform for listing events in Tokyo, you could for example search the events section and filter for public events tagged with Art in the Tokyo Area. Probably most the info is in Japanese only though!
Hope that helps