I recently became aware that the A.N.S., Internet Archive and the Newman Numismatic Portal have teamed up and made available digital scans of early Sotheby Sale Catalogues. (Pre-Tyssen to 1922, including Dimsdale, Montagu, Murdoch, etc.)
The scans of the catalogues, including those with plates, are very good and many have the added bonus of being named and priced; not the same, of course, as having the originals but a great resource for those conducting research.
Link to page: https://archive.org