I am looking for demographics of potential audience members of a professional writing portfolio.
Try a library database which will get you academic research on the topic. The ones that seemed to have the most material are Communication Source and Business Source Complete. You can get to them by selecting the Databases tab from the library home page. For both of these databases, try this search:
Into the first search box, type:
Professional writing portfolio
Into the second search box, type:
Audience or demographics
There aren’t exact matches to this search, but it comes up with some articles you can select that look promising.