1. Check on Craigslist under "Bicycle Parts > Tandem" and under "Bicycles > Tandem". Where I am located (SoCal) may not be the same as other places but I can easily find listings with tandems that are about to hit the dumpster or "projects" that people started and never finished. You can pick up frames, forks, whatever you need for cheap.
2. If nothing turns up under "Tandems" then look for "dirt jumper" or "dj" forks. One rigid fork that was officially "tandem rated" by the manufacturer was the Surly Instigator. However these don't appear to be made anymore and may be hard to find.
However if you can find a different dirt jumper fork with similar specs that might work out. For suspension forks, look for Marzocchi Dirt Jumper (DJ1 or DJ3). These forks are more rigid / stiffer than standard MTB forks and I think were "tandem rated" by the manufacturer at one point. These forks also don't appear to be made anymore but you should have an easier time finding them used on Craigslist, eBay, Pinkbike.com, etc.