Hi, we bought it at the border, normally there are several shops, supermarkets etc they have a booth where they sell it. There are two kinds of insurance. One is per day, the other is per year. If u are staying longer than 30 days, the year insurance is def. cheaper. Another good option which was introduced last year is a drivers licence bound insurance. It is really cheap (like 120 Dollar for one year, about 50 % compared to the normal one for a year), but than nobody else can ride yur bike (which was ok for us)
If u check with AAA they can give u infos as well. But the price does not differ much.
Martin and Katja