Who should I ask for recommendation letter after graduation?

After graduating for two years, I decided to apply for the Master program. I have prepared essays and scores but now I am stuck with recommendation letters. I have got 2 – one from the current manager and another one from my undergraduate institute. However, I am struggling with one last letter.

I came from a big public school (UC system) and for some reason, a lot of my upper-division classes were taught by instructors, not professors. And unfortunately, at least three of the instructors either left the department or left academia (got into the industry) and I am not exactly sure if I can contact them (even if I could, they are no longer in the department and that might be an issue..).

I have previously contacted one faculty that I have TA(undergraduate IA) for months ago. But that doesn’t work due to some complicated issues (the faculty’s schedules). My latest research volunteer experiences(short) were 4~5 years ago and it was extremely hard to contact supervisors as well.

Is anyone can give me any suggestions? I am living in another country right now, so visiting the campus is not really feasible. Understanding that Master’s program does not care about LORs as much as Ph.D. programs, I am worrying that I couldn’t have all the required documents.