@SillyFlowerOkay There isn't a way to buy them and the stories published on the website are the full versions. I believe they're intended to be equivalent in length to the bonus stories available in the premium epub (i.e., post-card sized, store specific stories shrink-wrapped with the book).
The stories themselves were perks for high tier backers. IIRC, the stories would've been kept private to the backer that requested the story, but all the backers allowed JNC to publish them on the website for everyone to read.