Before transferring a program or disk image through FTP, make sure to configure your FTP client for binary transfer mode. With most FTP clients you can do this by simply typing the word "binary".
ftp://public.asimov.net/pub/apple_II/
This site is the largest Apple II emulation site. Under the /pub/apple_II directory, you will
find disk images, utilities for making your own disk images, and
Apple emulators for other computers and operating systems.
https://archive.org/details/softwarelibrary_apple/
This site contains a number of collections of Apple II software. Including: