most aquarium shops have them as the fish are delivered in them. Also try chemists as sometimes their meds come in them. Also as sleepyD says, fishmongers, butchers (be prepared to scrub and scrub ). Theyre also on ebay, just be wary of postage.
Some places may ask for a fee, usually about £5. One shop near me wanted a tenner!!! I walked out. lol
even brand new they cost ~ I'm lucky in that one of the places that makes and sells them has their main factory about half hour away from me so I can order them and then pick them up .... which is just as well as the fee for them to send them is horrendous ~ is more then the cost of the box :gasp:
One of my local Reptile/Aquatic shops where i got my one from today does this thing where you can "hire" it for a fiver & return it for your money back or if you want to keep it you can :2thumb:. I got quite a large one which i hope will do me as an incubator for the 2010/2011 season :2thumb:.