Hmmm, these aren't common and when first available quite expensive - and still are if this site is still selling them (if so, it's no wonder at that price!!):
https://www.scandig.com/colour-mana...-it8-target-transparency-kodachrome-35mm.html
I am kicking myself for not buying one back when I purchased my 35mm scanner - would have saved me time getting K14 scans that were acceptable and as close as I could get to the originals.
As for any other forums that may be out there who can help, I had a quick search for IT8 Kodachrome targets and a few popped up, but many were posts for looking for the same as you are: an IT8 K14 target.
I think you'll be hardpressed to find one on a forum - I found one on the Macrumors forum dated 2012, but he had issues unloading it and ended up sending back to SIlverfast for a software upgrade . . .
Sorry not to be more help. I ended up tweaking what I already had in the SF IT8 software - it produces good results for my needs and if I had time, I'd definitely attempt to tweak further.