Go to the supermarket in the evening when they are restocking and usually you can get loads. Normally they flatten them but if you've got some parcel tape then it's no problem putting them back together again. It also means you can get more in your car! The ones at the front in B/Hill tend to be either wine boxes or cardboard trays used for fruit and veg and aren't too useful.
Supermarkets are a good bet. Give your local one a call in the morning first though - as someone who works for one it's best if they know they need to keep some otherwise they get put into a baler and crushed to be recycled - if they know to keep them it's less inconvenient. Usually some massive boxes (the ones for multipack crisps are usually huge).