School Slippers

It's pretty common and, as I remember, even mandatory to change into slippers at Czech schools.
Not only at school but most of the Czechs wear slippers at home as well.
I noticed this seemed to be kind of a strange habit for the Dutch, there was not much difference in Belgium, nor here in the U.S.

Till Maarten brought this note home from his teachers.
He should bring slippers to school to wear during class-time.
What a step towards comfort and hygiene! It can be a long day wearing shoes.

After school Maarten is used to going barefoot as often as the weather allows.
But he is going to start an extended day program beginning in March and the slippers may really be useful.

Any suggestions where to get them?!