I imagine you might find some apples leftover at an orchards warehouse. Consider waiting until apple season arrives in September/October. You will be better off using the freshest ingredients possible.
I'm not sure how it is in the UK, but here (at least in Washington) there is a huge surplus of apples in cold storage now. I think the paper said this year there is like 37% still in storage vs last years 25%. I guess due to the really good crop and the economy.