https://picons.xyz
Download which ever ones you prefer, I find the highest resolution .srp ones to be the best.
Extract the the folder to your hard drive some where and rename the folder to just picons.
Then in tvheadend config webui choose to use picons (picon path)and put in your location of your folder you extracted for example i enter this for mine.
file:///rob/home/picons
One other thing to add, that site has a few variations for example light on transparent, dark on transparent etc.
I have a dark theme in use for kodi so I choose light on transparent which make my channel logos a light colour on my dark theme :) makes them easier to see if you know what I mean.
If you get stuck just give us a shout
Rob