Where Can I Find Transparent Bag For Air Travel? (UK)?

It sounds simple enough but all the supermarkets have their logos printed all over the bags or the bags are tinted. Delta's website says I need:

"One clear transparent resealable 1 quart-/liter-size plastic bag (slightly larger than a sandwich bag)..."

I've also found it doesn't need to be that big, but it can't be larger.

This is for holding the liquids for my carry-on. Does anyone know where we can find such bags in the UK?

at uk airports thay hand them out in the queues to board - at least at heathrow and gatwick they do.

they hand them out at the airport free