German native speakers very often (almost always) don't speak exactly what you learn as Standard German. Standard German is a somehow artificial language, created from and for German native speakers, to be able to communicate with each other even if they live hundreds of kilometers away.
German has lots of different dialects. It always had. And it took many attempts and experiments to unify German dialects to one language, that every native speaker might understand. What you learn as »Standard German«, and what most of us native speakers have to learn in school, is the result of this process, that took many centuries.
Standard German is a unified and standardized language. It exists in three flavors: German German, Austrian German and Swiss German, but for you, on level B2, you can still consider them as one language, the differences between those variations are not very big. (The situations is similar to American, British, Indian and Australian English)
In Standard German, that has rules written down in books, the only one correct version of your sentence is:
Hast du einen Stadtplan von Düsseldorf?
But most native speakers speak colloquial forms of German, which has no standardized rules. (Of course there are implicit rules too, but nobody wrote then in books.) Depending on the geographic region, you might hear:
- Hast du ’nen Stadtplan von Düsseldorf?
- Hast du ein’ Stadtplan von Düsseldorf?
- Hast du an Stadtplan von Düsseldorf?
Of course you can't hear the apostrophe. I just wrote it to mark, that here is something missing.
In (1) the leading diphthong »ei-« is missing. I think, this is typical for middle and northern regions of Germany.
In (2) the last syllable »-en« is missing. But I don't know in which regions this is common.
(3) has emerged from (2). The leading diphthong »ei-« has turned into the vowel »a-«. This is very typical for southern regions (Bavaria and Austria).
But in all three forms, this still is accusative case. The speakers just use a shortened non-standard article.
There is also another Question where you read more information about this topic.