its not a very common item, to my knowledge...so it might take a few tries. i still cant explain it, but shopkeepers swap their inventory from time to time. not restock...swap. as in, you can come back in 30 seconds to 1 day and they could change an item to available and then gone. antone STILL does this with heaumes at his shop. i can see one on his list...sell him something...then go back to buy and it's gone.i unfortunately passed up the blumberbuss or whatever it is when i first saw it, and now i cant find it at any of the shops. 
good luck with the holy h20