Registered on here specifically to post this - I have been looking recently for somewhere to buy empty whisky casks from (looking for a wee project) and although the other posts on here are years & years old, still came up on my google search. I wanted to help anyone else in a similar situation in the future.
First place my search took me to was Ebay etc - do not buy from here, at the time I was looking, a barrel was selling for c. £130 PLUS postage. Not cheap. Even saw one go as high as £1085!?!?
After a few hours search, and a few phonecalls / e-mails I came up with the following:
1. Try your favourite distillery - you might get lucky, some distilleries have their own cooperage (e.g. Balvenie) and even assuming otherwise, inevitably have a few spare casks lying around. This is unlikely (although one of my friends lucked out and got one from his favourite distillery, but I've no idea about costs) and the distilleries I spoke to did not seem keen to sell their old casks for some reason...
2. Try a local whisky specialist shop - (luckily there are quite a few nearby) a couple of them told me they have (or knew someone who has) and old cask lying around and I could uplift for free (or very minimal cost for the cask).
3. Try contacting a cooperage - most make garden furniture or similar from the casks they will not or can not repair, and usually sell whole casks too. The bonus of this is they are relatively cheap (usually about £30, but depends obviously on size of cask) and (a big bonus) that they can usually arrange delivery for you, although this can get expensive due to size and weight.