I want to replace my father's head stone with one from the VA. What are my options?

I agree with Carol's answer. I also think it is important that you contact the cemetery first because there may be fees involved for removing the existing Memorial.

Also, here is a link to a section of the VA website that explains that refers to "Headstones, Markers and Medallions". http://www.cem.va.gov/cem/hm_hm.asp

First you can talk with the cemetery owners. If they can't help you, there is likely an organization in your state that oversees what they do. Try your state website and type keywords in the box to see what they have. Another idea is to talk with some funeral directors in the area. They are generally up to date on these issues.
Good luck. That marker means a lot.
Carol