Quite a lot of the books you list aren't available as ebooks, partly because a lot of publishers haven't yet got round to putting their back catalogs into ebook format yet. But the following are definitely available:
Niven -- The Mote in God's Eye, Footfall, Lucifer's Hammer are available from Baen Books
here
Pratchett -- Most (all?) of the Discworld books are available at BooksOnBoard,
here. (Look for the ones with the Mobipocket icon).