Her tickets are always sold through tickebo, which accepts international credit cards and international mailing addresses. All you need to do is sign up through their English site, rather than the Japanese one. (Your account will work on both the English and the Japanese site though. Don't worry.)
On a different note, for anyone wanting to join the fanclub to get the personalized fanspace edition of "Finally"... Those orders are being handled through mu-mo, which does not offer international shipping. So you'll have to use a forwarding service like Tenso to get your album when ordering.
