Go to
www.abandonia.com for old games. Unfortunately most of Sierra's games are ESA protected, so you can't download them from Abandonia or anywhere else for that matter. You may be able to find them by digging around the internet though.
Abandonia has tons of old games though, and they have instructions on how to use all of the different progs you may need to get them working. I don't know of any sites to d/l Sierra games, but hopefully you might find something you like at Abandonia.