When working on a fictional personal project that will only serve as a portfolio piece (i.e. briefbox projects), how can I find the appropriate data to create user personas? I want to emphasize my process but I feel that the lack of data and no client or real users are a major road block.
I would suggest that persona's and empathy maps are MORE important than the final product at showing your process. That said, if its a fictional project for fake users, I see no problem with you inventing some attributes.
Look at it this way, you're getting an opportunity to really practice empathy based on your experience with a population. Alternately, you could make up a fictional project to address a problem that real people who you know (family, neighbors, friends, co-workers) experience and then talk to them about it.
Plus you're gaining a deeper experience into the actual design process. So you should be documenting this struggle and using to help tell your portfolio story.
If you have no interviews with users, you cannot make personas. At any time. Personas should never be based on anything else.
However, if you have some data, safe assumptions idea of user archetypes, you can make some proto personas instead.