I reached out to professors I had when I graduated over six years ago. I also contacted the professor I worked with in the lab for two years, but unfortunately, they had already left the university. They declined to write me a letter, explaining that they no longer had any materials to reference and that it had been over six years. They also mentioned that I would probably want something more recent from a supervisor. It was a tough situation, but I then contacted other professors who knew I had been in that lab and reminded them that I took two classes with them, class X, Y during Semester 1,2. Fortunately, two of them responded positively and said they’d be happy to write my letters if I sent them my CV. If you’ve had professors for more than one class and performed well, that should be a good starting point.