The Psion Software Index is the easiest place to find software.

There are also a number of sites with links and downloads that might be useful, and the Internet Archive hosts copies of a number of publishers’ sites that are no longer available anywhere else online.

Here’s a few we’ve found useful:

• 3-Lib – 3-Lib was always the source for Psion shareware, and Steve Litchfield still sells copies of the most recent 3-Lib library CD, physical or digital.
• freEPOC (Internet Archive)
• Neuon (Internet Archive) – Neuon published a sizeable number of Psion apps and released freely available license codes for many of them at the end of their life. While some of these apps use native libraries (and therefore won’t work), we’ve been thoroughly enjoying games like Chain Reaction and Tile Fall.
• 5mx Software in the 21st Century