They are listed in the current DMM Trade Pricelist (ref A6101, SRP £23) so any DMM stockist should have no problem getting hold of them for you, assuming you are in no great rush.
I have a similar issue for my foot fangs.
when I was on an avalanche course in Scotland last year, one of the instructors suggested using a car inner tube.
I have the inner tube but haven't tried it yet.