User research without access to the users

I wonder what should I do as UXUI designer when I have a project where unfortunately I do not have access to possibles final users but I really need to do a user research in order to start building my personas etc etc.

At the moment I using my experience in similar projects, some user reviews from other similar products I found on the Internet or competitors products and finally my intuition but I feel I am taking shots in the dark.

What should I do? What is the process in this case?

Edit:

Some extra info: "It is a new product (an architecture portfolio) I have tried to speak with my stakeholders and I only have "might be" and a lot of "hmmm... " "