I want the list of Stack Exchange sites that support a tag, let's say sun. How to find a list of all sites that have the sun as one of the tags, instead of searching in all individual sites?
I've modified the SEDE query I mentioned in the comments slightly; this new SEDE query should do the trick. You can click on the entry in the 'Site' column to go to the tag info page on a specific site.
You immediately see 'sun' means something different on computer-related sites than it does for science, as @RobertLongson noted.
An easy way to do this is to use the filters page (although this probably wasn't its intended purpose):
Just select "Just questions tagged with the ____ tag", then "Just these sites ______". You may have to also "show more sites".