If you, nor anyone you are with drinks alcohol but you are still paying a seller of alcohol (by buying food, non-alcoholic drinks etc) is that permissible?
And let there be [arising] from you a nation inviting to [all that is] good, enjoining what is right and forbidding what is wrong, and those will be the successful. http://tanzil.net/#3:104
... do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty. http://tanzil.net/#5:2
It is obligatory for any Muslim to forbid what is wrong. Forbidding has different steps. The lowest steps are any action that the seller understand this is an evil action. For example by fret or by not buying or by any other similar action. These are lowest steps of forbidding what is wrong (in view of Islam). Buying from such a seller has conflict by this command of Quran. There are next advanced steps that need more power.
Also buying anything from a seller is a kind of financial supporting and cooperating him in his business. And because cooperating in sin is banned in Islam so financially supporting a sin business is forbidden.