As Captain2 says, auctions are a great place to buy stock.
Some auctioneers specialise in selling bankrupt stock whereas others might have things that you're interested in amongst general household sales. (The former have the advantage of offering plenty of stuff that you might be interested in during a single sale but that also means that there will probably be other traders bidding against you. The latter have the disadvantage that you might have to spend hours there just to buy a few lots but the possible advantage of you being the only person interested in those lots).
This auctioneer, for example, is a specialist in liquidated stock:
http://www.johnpye.co.uk/all-auctions/
whereas these auction rooms (where I'm a regular visitor and occasional purchaser) mainly has house clearance items, etc but often includes some trade stocks:
http://www.thesalerooms.co.uk/General%20Auction/General%20Auction.html