reMarkable is offering an official sleeve aka. Folio, but many users aren't very happy with it's quality. This page tries to list alternatives.
When looking for an alternative, it's useful to know that the following more popular devices have roughly the same dimensions as the reMarkable:
CAUTION: Do not buy any cases that have a magnet in them. Magnets impact the EMR Pen technology and will temporarily impair the functionality of your device. If you do bring a magnet near the reMarkable, a few reboots will clear the issue.