There was an exhibition of a lot of his paintings at the San Francisco museum of Modern Art - they probably have a website/email, so they sould be able to help. BTW the exhibition was fabulous and had a lot of background material on the different themes and motifs in the paintings.
Almost certainly the National Gallery in Brussels. There is a huge permanent collection of works by Rene and Georgette Magritte in the Modern Art section of the gallery.