I've seen them for sale before. Look at anything made by McNeills / Highbarn too they are very similar I had one I sold last year it was great. Ian McNeill is the father of the guy that builds the Peper Harow boxes so expect he learnt his trade off him. I don't think he builds anymore though.
I've got one - bought it last October/November. One came up on an internet search, being sold on behalf of the owner by Peper Harrow, so I contacted them direct and they had a couple for me to look at. It's worth e-mailing them with your requirements.