The only thing close I could find was a Jonathan E. Mitchell Charitable Trust
in Beverly Hills in the 1995 edition of the Foundation Directory.

While the directory indicates that this trust's fields of interest are
"education; hospitals (general); medical research; jewish agencies & temples,"
I note that the list of grants includes $1,500 to the Los Angeles County
Museum of Art.

Notes indicate that applications are not accepted and giving is "primarily" in
California.  I don't see a phone number; the address is c/o James Turken, 9300
Wilshire Blvd., Suite 555, Beverly Hills, CA  90212.

Good luck!