Could you specify a particular region? I'm assuming Europe, but it probably makes sense to consider Eastern, Central/Western, and Southern Europe separately.

You may be interested in this article. The answer is essentially everywhere. Having a large network of potential buyers and sellers is essentially what makes being a merchant work, so there were Jewish merchants everywhere. That said, a Jewish merchant wouldn't need to visit every town to get that effect--merchants held fairs, in which a large number of merchants would conduct commerce in a central location for trading. So a Jewish merchant in the Rhine Valley probably wouldn't be frequenting towns in southern France, but another Jewish merchant might.