The Super Tenere has just been imported to the U.S. in 2012 so would be mainly available at dealers. Actually, they started importing the 2012 model in summer of 2011, so there may be some used available. There are no older pre-owned model years of the Tenere in the U.S., as it wasn't EPA approved until this year.
The preferred place to find used motorcycles in the U.S. is online at craigslist.org under the motorcycle sub-section. It is basically an online classified ads and has good used bikes available from private parties with pictures.
The craigslist site doesn't allow a global search of all cities in the U.S.
To do that you have to type:
site: craigslist.org: tenere
in the Google search box and it will return all the pre-owned Teneres for sale on Craigslist in the U.S. and you can figure out which state is a possibility.
Probably easier to find a KLR650, DL650, or DR650 if you need reasonably priced mid sized reliable used moto in the U.S. Used dual sport Yamaha choices are limited in the U.S. Although the WR250R would be my choice. Fuel injected, bullet-proof motor, great fuel economy, able to cruise at highway speeds, available used and in good condition, great on backroads and on metal (as the kiwis say). Just a thought.
I did notice that a nice Hubber named Wheatwhacker has posted a few weeks back that he will be happy to help foreign buyers find a bike and help them figure out how to get it registered. He lives in San Francisco, California and you would have to search this site to find his post.
I enjoyed my travels in NZ. If a slow person like me can find wheels in Christchurch and get it registered and insured and tootle around your lovely country, you should have no probs here. And of course, if you are travelling through rural northern Nebraska you and your mates are welcome to stay in the guest room here.