Hey! I believe they're all sold out, I'm afraid. It was a limited run and all the copies are gone.
There was a small re-issue* last year by Heimat der Katastrophe. It might be worth keeping an eye there in case it gets another run.
*I say re-issue; the music's the same on the Heimat tapes but the whole thing's presented as the Thousand Empty Light soundtrack (not Sleeping in a Drowning Stone) so the packaging is different to this one.