Borrowing is certainly your better bet. Ask your friend if his insurance policy will cover a licensed rider from another country. I asked my insurance (State Farm) about this and they said such a friend would be covered under my policy if they borrowed my bike. Be sure to get an International Drivers License to avoid any hassles.
Buying a bike is more complicated because then you need someone who can register it for you. If you want the bike in your name, then it will depend on the State how easily that is done. Some states require residency, others don't care, and yet others require a US Drivers license before you can register a vehicle. So, you'd have to check with the Washington State DMV for example.
The other alternative is to ship your own bike over. I've seen a group of BSA riders take advantage by shipping all their bikes in one container, and their UK insurance was good over here, so that package deal worked out well.
Better to be an idiot than a crook.