Temporary insurance can be purchased. If you are visiting the US and wish to have insurance for a short time you can purchase coverage for 30, 60 or 90 days. The coverage options are much the same as any other policy. It is best to plan ahead as it may take a bit longer to get these types of policies in place, and if you are touring you will want to have the proof of insurance with you. Typically they can be paid for with a credit card.
We often hear from clients that come to the US to purchase a motorcycle, and then tour on the bike. At the end of their holiday they ship the motorcycle back to their home country. It is always best to protect yourself from liability arising from an accident, and to make sure your motorcycle is covered as well.