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.