I was out of school for 5 years and found this to be the hardest part of the application process. I was however, able to track down some professors to ask for LORs. The ones I asked happened to keep records of all the classes they've taught, so they were able to verify I took their class and what grade I got. Surprisingly, one actually remembered me even though I didn't really go to that many office hours. The others that didn't remember me said they could write me a rec, but it would be pretty generic but considering I did well in their class, they were willing to at least make it sound positive. Make sure to prepare a resume and personal statement for them to read when you meet with them to go over the letter process.