Some of my personal favorite filmed Ian McKellan performances include:
Richard III (1995)
starring Ian McKellan as Richard III
directed by Richard Loncraine
(currently available for instant streaming on Netflix)
Great Performances: King Lear (2008)
starring Ian McKellan as King Lear
directed by Trevor Nunn
(currently available for instant streaming on Netflix)
Other performances available on DVD (also through Netflix) that I have not personally watched include:
Macbeth (aka A Performance of Macbeth) (1979)
starring Ian McKellan as Macbeth
directed by Philip Casson
Edward II (1970)
starring Ian McKellan
directed by Richard Marquand
Available for sale on Amazon.com:
William Shakespeare: Othello (DVD release 2004, I believe the performance is from 1990)
I hope this helps! Amy may know of more, so if you’re still jonesing after investigating those, drop her a line!
Enjoy!