I read primarily Science Fiction and Fantasy books, and have found a steady selection of ebooks for years now - if anything, I'd say the SF/F genre is better represented them some others (though I almost wish I was into romances given how much Harlequin has gotten into the ebook game). That said, the older, still-in-copyright books you will probably have a hard time finding - but that is true of almost any genre. I'd definitely give Baen a chance - you can download a ton of free books that they have made available at
http://baencd.thefifthimperium.com/, as well as the free library section of
www.webscriptions.net.