Good luck hunting a pair of those down. If you're open to alternatives then late 80s Diamond Backs came with 1" straight legged forks, but 100% appreciate they are not the same.
I have a Slingshot too and really looking forward to getting it build up so I can ride the thing as I'm super intrigued to see how it handles! Good luck with your hunt!
And stupidly forgot to mention other straight legged forks in 1" flavour: Kona Project 2's - possibly the closest looking to the Slingshot too. MS Racing - had straight forks on their 1989 range before they became Alpinestars