My company also faces the problem of supporting legacy ISA cards. They are part of our PC based automated test equipment on the production line and to replace these ISA cards would require an expensive system redesign. We found this local company with sells computers with both ISA and PCI slots: **broken link removed** .
It is a back-plane with ISA and PCI slots driven by a single board computer card. You can use PCI and ISA cards simultaneously.
This has worked well for us. We can replace outdated, sometimes unreliable, computers running old versions of Windows with modern machines/OS and use the existing interface cards. We have occasionally had to modify our application because the new machines are so much faster than the old ones and the old application wasn't written robustly enough (or correctly).