I have been out of school for a long time, and I'm not sure my professors will remember me. Who else can I use as a reference?

Consider who is in a position to "make an appropriate assessment of your academic or professional competence".   Who could address your abilities to do academic work?  Have you taken (or taught) any classes recently?  Have you written grants or other technical documents?  An employer may be able to address your professional competence, or possibly a volunteer coordinator if you have been doing volunteer work.   If you have questions, contact the Program Coordinator.