Math Symbol for "Where"

In his famous paper How to write mathematics, P.R. Halmos says the following about "where"

"Where" is usually a sign of a lazy afterthought that should have been thought through before. "If $n$ is sufficiently large, then $|a_n| < \varepsilon$, where $\varepsilon$ is a preassigned positive number";

That being said it is common to write

$x = 2^n$, where $n$ is some positive integer,

although I would prefer

$x = 2^n$ for some positive integer $n$.

Now to answer your question, there is no specific symbol for "where" that I know of.

There's not really anything that makes a symbol "official" or not, and there are probably authors out there somewhere who do use a symbol of their own devising for this.

However, there is certainly no widely used and understood symbolic way to write what you want.

Mathematicians in general seem to think that using prose (such as the word "where") for this is preferable, in the sense that the extra space taken up by it is a lesser cost than it would be for everyone to have to remember a specific non-verbal symbol.

The use of natural language is often more effective when presenting an idea. In my opinion the less often a symbol is used where a few words can go, the better.

That said, there are many places where symbols are useful and simplify matters.

The word "where" can often be replaced with "such that", and corresponding to this we have a few regularly used symbols.

For instance, in set builder notation the colon (or bar) is used to indicate a condition (read "where" or "such that"):

$$\{ x \in \mathbb{R} : x > 0 \}.$$

Also $|$ and $\ni$ are used for this same purpose.