This one's been problematic. Tony George (teejee) maintained it really well for a long time. But since then, the maintainer has bounced all over the place.
You might try this ppa
sudo add-apt-repository ppa:tomtomtom/conky-manager
But honestly... I just built it from source. It's easy - and building will also get you extra useful stuff.
How to build:
sudo apt install build-essential git valac libgee-0.8-dev libgtk-3-dev libjson-glib-dev gettext libgettextpo-dev p7zip-full imagemagick
cd conky-manager2
make
sudo make install
BuhLEEV in yourself. 
You can do it.