Unfortunately, 26" cantilever forks are getting harder to find. Harder still are tandem-rated forks. However, you should be able to find something. Assuming you have 1 1/8" threadless, you need to search for the specific dimensions you need:
26"
1 1/8" threadless, XXXmm steer tube (or longer uncut)
Cantilever bosses
XXXmm axle-crown (suspension-corrected rigid forks are longer than old school rigid mtb forks)
XX offset (you may not have many options here)
Material? You'll be choosing between steel and carbon fiber. Cannondale used to spec their mountain tandems with their Pepperoni aluminum forks. Not sure if the latest vintages were spec'd with this. So given this, you'll have options: one possibly reasonably-priced and the other stratospheric.
Wound UP Composites doesn't show a 26" fork:
https://www.woundupcomposites.com/pr...x-canti-1-1-8/
$515.00
Alternatively, you may simply want to call various tandem manufacturers to see what they offer.
Tandems East has steel forks for $275:
wwTandems East Forks & Handlebars