Here we go:
http://filmwasters.com/forum/index.php?topic=2345.0BTW, the title of your post is a bit confusing, at least to me. To the best of my knowledge Polaroid never made any LF pack film....although even as I type that I can feel people getting ready to correct me, so maybe I'm wrong. Not since I've been using instant film they haven't anyway.
However, it does seem strange that they'd make a back for someone else's film.

ANYWAY, the Fuji own-brand back is called the Instant Holder PA-45 and you might see if Skj can track one down for you in the non-traditional Tokyo which seems to love his camera so damn much. They've been on sale there for years, but the BJP announcement is about them re-introducing them to Europe. I haven't looked up how much they cost, but I doubt they're cheap.
I got my Polaroid 550 back on evilBay about 5 years ago for around GBP40 I think.