a shop or the shop

[ph0enix]

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?

[księżycowy]

I wouldn't bat an eye at that usage of the indefinite article.

If someone said this to me, I would think that there were more than one shop in that area and that the ticket could be bought in one of them. By saying "a shop", I wouldn't think that you could get a ticket at any shop, rather a particular shop amongst a few.

[Dormouse559]

I agree with księżycowy. Additionally, the indefinite article can be used when the speaker has a specific thing in mind but doesn't expect the listener to already know about it or to be able to derive it from context. So it could be that the speaker in your dialogue thinks the listener doesn't know of the shop (reasonable, since they have to ask where to get an Oyster card). It could also be the case that the shop isn't immediately visible, so that the speaker could point it out, for example.

[digitalmarketingboy]

It ca be both ways....

[vijayjohn]

Yeah, basically, a shop here means one of the shops.