I spent quite a bit of time looking into Tack Lockers recently.
You can get a police locker (60cm by 60cm, or it'll be too small) for circa £150. Cheaper than the tack locker of the same price, although the Police locker doesn't have the associated hooks and racks, they would be easy enough to put in. As an aside, I would be tempted to add one of the racks which takes a padlock.
I did contact West Yorkshire Police trying to track down some second hand lockers, but no such luck unfortunately. I'm sure there is a contract for their disposal somewhere, but nobody knew of it.
Since Daffy retired my tack has been kept at home, but if I do see a second hand tack locker locally, I'll probably snap it up for the convenience of having one.
ETA, I got my quote from Link Lockers. There was some negotiation over price, and if you could find some other interested parties, you might be able to get a discount. The things that bumped the price up to £150 were VAT and shipping.