Where can I purchase a battery (E12) for Harbor Breeze remote model KUJCE9805?

Have you tried CVS drugstore? THey have a very good selection of Batteries, and if you are in the USA they are literally everyplace.

Lowe's, it is their brand. Most remotes takes standard batteries. Open the case and look.

Try these guys https://www.batteriesplus.com/battery/digital-camera