I encountered the following dialog in one online English course:
- Do you have an Oyster card? You cannot take a bus without it.
- No. Where can I buy one?
- You can buy it in _a shop_ over there.
Now I have doubts about "a shop". Shouldn't it be "the shop"?
Because it is obvious that there is a concrete shop, where tickets can be bought. It is highly unlikely, that there are multiple shops, any of which will be ready to sell tickets.
Or could it be either one, depending on the context?