Sorry that this is not a response to your question, but I was just about to post exactly--EXACTLY--the same question on here. Except in my case it was my MA that I finished 10 years ago, and I am applying for a PhD in Public Affairs.
My graduate school courses were large, so it was difficult to build relationships with professors. Even when I was enrolled, I doubt that they would have been able to write meaningful recommendations. I have considered contacting my professors, stating which of their classes I took, and including copies of graded papers that I handed in. However, I after extensively searching, I can't find any papers that I handed in, and without that, contacting them does not seem particulalry fruitful.
I can't imagine that other people don't have this problem. It seems silly to let this one thing keep me from applying, but I'm not sure what to do. Has anyone else had a similar experience?
Thank you!