Looking at the availability on their website, weekdays have tons of availability. Weekends and holidays are quite full though. It'll be a lot easier to just buy it in Japan.

If you want to buy it in Japan, go to this website:
http://l-tike.com/fujiko-m/

Click on the part with the random machine (looks like a red ATM) and 3. Print out the page and just bring that part. Write the date in numbers (so April 18th becomes 4/18). Go to any Lawson in Japan and show it to the staff and they'll help you. They're not going to complain or hassle you about this unless the staff is rude and the place is busy.