In reply to Jenny C:
If, as you say, the condition is widespread, perhaps there's a need for some kind of on-line 'swap centre'.
Then again, if the larger foot is invariably on the left (most people being right-handed), it wouldn't be of much use anyway.
Another idea is for rock-shoe manufacturers to start selling shoes individually: I know Evolve were advertising odd pairs at around a 20% price premium a few years back, but I don't think this was ever taken up by the UK distributors.
Pete.