Somewhere on the Internet Archive is a series of BeOS ISO images, but I refuse to link to it.
HaikuOS is a good OS to try and use BeOS API calls to make it work like BeOS.
Do you know how to get it running on vmware/virtualbox? I spent like two afternoons once trying to get them run but could get past the booting stage and just kinda gave up.
This may have been in the past but it's straightforward now. I tried it out maybe 6 months ago and had no issues.