I too live in Edinburgh. Unfortunately I know of nowhere in the area. I will ask a joiner in Leith who uses quite bit of it and see where he gets his and how it is delivered. I will pass that way in the middle opf the week. Best wishes.
When Brodies timber, previously Burhouse, up near Dunkeld opens this month, they night have what you're after. a bit of a trek from Edinburgh, but may be preferable to mail order.
