You should check with JTB in the US. I purchased a ticket for the Ghibli museum in Tokyo through them and it is also one of those tickets you can buy at Lawsons 3 months in advance.

I'll link to the site I bought my Ghibli ticket from since it gives the phone numbers and e-mails of their offices. Doesn't hurt to check. (I bought mine from Toronto since there was none close to me and they sent it my mail).

http://www.jtbgmt.com/eng/ghibli/TicketSystem.html