A quick google search for “jrun-31-win-us.exe” showed a couple of places you could download it from. Additionally – you could just do the trial from Macromedia site (the developer – non purchased version allows up to 3 concurrent connections).
If you want any help let me know – I know alot of companies (unfortunately) are still using JRUN3.1 as the internal supported system…
Thanks,
-K