How can a freelance writer supply recommendation letters? Suppose, a BSc in Computer Science student has been doing online freelancing for the last 10 years. The teachers who knew him in his undergrad school are either retired, or dead, or doesn't have any contact with him.
Where can he collect recommendation letters if he wants to get enrolled in a graduate school?
Freelance workers can get reference letters from clients who are repeat customers over many years.  Business partners could also write letters.  If you are participant in any industry organizations, colleagues in those organizations could write letters.
If you do not have any of those relationships, you probably cannot get a letter without forming new relationships.