Check the NATRC website.  http://www.natrc.org/  They list all the sanctioned rides and clincs.(not what you are looking for, but a starting place) You might strike up a conversation with of the regulars about training and practice rides. Or talk to a ride manager about helping out.

Also check out the HorseCTR List at http://sports.groups.yahoo.com/group/horsesctr/

It cost so much money to put on a ride, and the sanctioning cost is such a small portion of the total cost. That I'd be surprised to see many rides that are not sanctioned. It's just not cost effective. The cost of a ride is the Vet & Horsemanship judges, liability insurance, Forest Service  fees, ribbons etc. Not the couple of dollars per rider that NATRC gets for sanctioning the event.

Go and volunteer to get your feet wet. Observe what the riders do and go home and practice. And the best way of all is just sign up as a Novice and go ride. Everybody will help you. After a ride or two you will be an old hand at it.