I reached out to professors who knew me well in college. One of my very best letters (or so I was told at my interviews) came from a professor whose class I took 6 years prior to applying. Don't underestimate their memories.
Alternatively, if you have a GSI or TA who knew you well and can attest to your personality and work ethic, I would ask him/her to write the letter and have the professor co-sign.