Glengoyne sell them outside the warehouses across the road from the distillery. There's a cooperage in the east end of Glasgow called Fishers that would no doubt sell them to you too. I expect there would be other places.
I occasionally see some at garden shops around here for about $75 (actual whole ones). They're Canadian. I would expect bourbon barrels to be expensive, since there's no such thing as a worn-out one--they can only be used once, and I should think the Scots would take all they can.