where can I buy a universal liner?

Does anyone know where I cam buy a universal liner for a older style birth pool in a box - I've got the old patterned one which looks a bit like a giant kids paddling pool, and I don't think the liners currently sold for the regular and mini models will fit. I had a universal liner when I used it first time which I bought with the second hand pool, but I can't seem to find out where to buy one now for my second birth!
Does anyone have any ideas?
Jo x