Why Are Some Japanese Erasers Always Hard to Find?

The availability of erasers depends on a couple things. The two producers of the Japanese erasers have different abilities to produce erasers. Iwako, the bigger of the two, has been expanding production as to keep up with the demand in the U.S. The other smaller producer, Zensinsyoji or TRC, has not ramped up production so their erasers are often harder to find.
  The purple cupcake eraser pair, pictured, is made by Zensinsyoji. They have not increased production of these erasers and are becoming harder to find.
  When we, as retailers, order erasers from the US importer of Iwako and Zensinsyoji, some styles of erasers come assorted in a master box of 50 or 60 erasers. So that may mean in box of 60 erasers we will get only handful of a particular type of eraser. My favorite example is the onigiri pair and grilled onigiri pair made by Zensinsyoji.  In the box assortment that they are included in, we get only 3 of each out of a total of 50 Japanese food erasers. We are always out of stock ,of these cute food erasers. We try to special order such hard to find erasers, and last year we had success doing that, but this year with the demand, Iwako and Zensinsyoji have been unable to fulfill special orders from us.
  Many of the erasers come as one type per master box. Those erasers tend to have better availability.