Here’s an archive of Siskel and Ebert’s Sneak Previews show at IMDB
https://www.imdb.com/title/tt0075583/videogallery?sort=date&sortDir=asc
I’ve checked out a few of the early ones. Your mileage may vary. Good stuff though.
UPDATE –
The IMDB archives seem to have gone away.
Try here: https://www.siskelandebert.com/episodes