Robert White have a reasonable stock which I saw on Tuesday. I too have been worried about how long they will be produced, so I bought one there and then!
Current Sterling vs Dollar will work to your disadvantage though.

The number of alternatives are dwindling too. These all fit into the all singing and dancing fiddly school.

The Pentax reads about 1/2 stop higher than my Sekonic 508, which should account for the large amount of push processing I have needed! Need to test this with film now.