This leads me to
this but I'm not sure if you would need
this one or
this one. They're both marked as backordered, so you might as well reach out to them to figure out if/when they'd ever get more, in addition to figuring out which one is more suitable for an F35. Saab did some wonky things with F35 diffs in F25 transmissions that I haven't been able to track.
If you're open to other options, the price of a Quaife + fitting the diff in an F35 is likely to be much more expensive than finding an F35 with the LSD already installed.
Alternatively, you could pick up a new or used F23 transmission for much cheaper, and for which you can buy a Quaife much more easily
here. You would only need a different front trans mount and shifter cable bracket compared to the F23.
I'm not sure where you're at in your build (update your signature
here!) but I think your frame is up for delivery soon based on a brief dig through your post history. If I'm wrong, DF can build your frame to fit an F40 transmission, but for that option with a Quaife installed, it's probably at least double the price of a 0 mile F23 + Quaife LSD installed.
edit: if/when I can confirm my F23 swap is solid, my LSD 4.05 F35 is probably going up for sale