Apple - Where can I find software for the Mac Classic?

Apple's website has a page dedicated to Older Software Downloads for systems earlier than 8.1.

(They also still have a support page for OS 9, but the Downloads link on this page just links to the current OS X downloads page.)

In addition to AppleCare's Older Software Downloads list, there are a few other sites that host old Macintosh software, including Software for Classic Macs and Info-Mac Archive.

Wikipedia's list of old Macintosh software should also help you get names of specific applications.

Macintosh Garden is another good source.