Here’s what I found out:
While you can reserve tickets through Lawson’s website (if you read Japanese), you have only 2 days to print your tickets out at a Lawson’s branch in Japan before they cancel your reservation. So it’s not a great option if you want to buy tickets in advance. It’s March 30th now, and the April 9th 10am slot is already sold out.
On the other hand, JTB only has a limited number of tickets it can sell. So just because you see open slots at Lawson’s website, this doesn’t mean you can get a ticket for them from JTB. I tried purchasing tickets from JTB on March 27 and they had no tickets earlier than April 21.
In summary, if you want to ensure a visit to the Ghibli museum, bite the overhead and reserve it through JTB, about a month in advance (probably earlier if you plan to visit during Japanese holidays like Golden Week or Obon).
One positive for buying the vouchers beforehand from JTB is that they are locked to a particular day only. The Lawson-bought tickets are tied to a day and time.
Credit:stackoverflow.com‘