There are some states that allow people (non-citizens/non-resident aliens)to buy bikes and register them. You usually have to have a physical address, such as a fellow Horizon member's. However, I know there are some states that do not allow this, such as Ohio. Keep in mind that there are 50 states, and 50 different laws when it comes to vehicles, as each state maintains it's own set of laws and regulations, rather than a blanket federal US law. Looking around could be a good idea, such as what Road Hog mentioned, but since the US is such as huge place, you probably would end up spending just as much getting to that state, as the registration/taxes would be on that bike. I know California doesn't have the lowest taxes, but it is probably far easier flying in to California, such as L.A. or San Francisco, finding a bike, and getting your journey started rather than looking for a bike in another state and getting there to buy it. Keep in mind the laws of economics as there are plenty of bikes in heavier populated areas to choose from, and thus, you can negotiate on the price, rather than a rural state with lower taxes but slimmer pickings on bike choices and prices.
With all that to mull over, what city do you plan to fly into? That will make it easier for us to guide you in the right direction.