There are vendors listed for both Moab and Green River. Can't really say which is best, but you should be able to find one that can accommodate you. For information about what 's needed, and a link to listed vendors, see: http://stateparks.utah.gov/stateparks/ohv/out-of-state-registration.html
The only thing I'd add to the above is that I-70 Exit 129 (on the map at above link) was changed to Exit 131 a few years ago. Get a copy of the "San Rafael Motorized Route Designations" map. It's free, put out jointly by the BLM, Emery County, & Utah DNR and is a very good map of the San Rafael Swell area. Have fun... 5MOH is really a blast on a trials bike. I usually carry 2 MSR 33oz fuel bottles, but could get by with one for my 4RT.