How does one find an advisor for an online graduate program?
I think it depends on the nature of the particular online graduate program. Unfortunately, for the reason RoboKaren suggests in her comment, I think that many faculty don't care for online education. Which faculty members recruited you to come? Which of them answered your questions when you applied? Which are teaching your classes, or other online classes available through the same program? These are the faculty members that have made some investment in this program, and I think they are the ones most likely to be willing to supervise you.

Three more pieces of advice:


I strongly second RoboKaren's office to travel to the campus and meet with as many professors as you can. Let them know that you're an online enrolled student and that you're travelling 6 hours each way and hope to meet with them. Knowing that you're going to such an effort will probably lead some of them to take you more seriously.
Don't ask anyone to be your advisor on the spot, or over an initial e-mail. It is a major commitment, which most people wouldn't make to a stranger (whether online or not). Wait until you know each other well and the professor is somewhat familiar with the quality of your work. In the meantime, start smaller. For example, if one professor recommends a particular paper for you to read, read it and ask some thoughtful questions. This has the chance to make a very positive impression.
If this goes poorly, you might consider withdrawing and enrolling in an in-person graduate program, if at all possible. 


Good luck to you.