References for Ph.D. admission application
You will certainly want to track down old professors. It might be helpful to get letters from professional colleagues, depending on your field. (I am in mathematics, and in pure math this would not be helpful. But maybe, e.g. CS would be different.)

It would be very helpful if you could get a newer recommendation from a professor in the field in which you would like to study. There are a couple of ways to do this:


If you live near a research university, find a graduate course that looks interesting and ask the professor's permission to sit in on the course. Do all the work assigned for the course, and do it damn well. Ask the professor if he/she would be willing to give occasional feedback, and/or grade your homework. If all goes well, you can ask for a letter of recommendation.
If this is not feasible (perhaps you live further away, or your working hours are not flexible), plan to do a lot of self-study and find some research university which is semi-close. Ask a couple of professors if you could make appointments with them, and solicit their advice on self-study and preparing for grad school.

If one is especially friendly or helpful, ask if he/she could later evaluate the results of your self-study. Go work your butt off, then return and talk to the professor, demonstrate that you've learned a lot, and let the professor teach you some more. Repeat if there's time. Eventually, ask for a letter.


Both of these depend on having the time to informally begin your studies early in a serious way -- but if you want to get a Ph.D. this is a good idea anyway. These ideas also depend on the generosity of professors -- this will likely be forthcoming if you demonstrate that you are serious, and also that you intend to be very respectful of their time. 

Good luck to you!