The Eets community URL, where one should be able to share Eets levels, is sadly not working. But there's a zip file with extra levels right on the main page:
http://eetsgame.com/downloads/EetsLevelPack.zip
The zip file contains one folder "levels" and that folder has a bunch of .eet files. One can just drop the .eet files into the Custom Levels folder and play them through the Puzzle Maker in-game.
The Custom Levels folder is in:
c:\Program Files\Steam\steamapps\common\eets\User\Custom Levels\