For piracy addons I've generally relied on word of mouth, someone mentioning them using a certain addon and me Googling it, but I think Quasar (torrent streaming addon) is listed on the Kodi forums (as it doesn't come with the piracy enabling part included) and that's really all you need most of the time since pretty much everything is available there.
But the simplest setup is with Quasar Burst which does come with piracy parts built in and that version you'd probably have to find elsewhere, I think there's a dedicated website for it. A couple of shows I've watched I wasn't able to find torrents of, for those I used Genesis, but I believe Genesis is no longer being maintained and there are new replacements for it, so you could just google something like "Genesis replacement kodi" or "Genesis alternative kodi" and you'd probably find something.
Also, there are the live TV streaming addons, these aren't technically piracy (although depending on where you are in the world actually using them may be considered piracy as they are often region locked requiring a VPN or proxy and even when they aren't they are supposed to only be used in certain parts of the world), so those you can easily find given that you have the correct repos installed.
Have a look at SuperRepo, it's a huge repo where you can find most anything, along with 3rd party repos for addons that aren't included in SuperRepo itself. There's a ton of stuff not available in the Kodi repo itself.
TVAddons.ag is another repo I've used, this one seems to be more focused on Live TV streaming addons for various countries, and helper addons for them, there may be something useful in there, however I don't really like their addon downloader, which I think you have to use as the addons won't show up through the normal Kodi addon downloader interface. That may be a good thing as it helps you find everything related to live TV streaming in one place rather than scattered across multiple categories that are sometimes not so obvious where you have to look to find a particular thing.
Also, I don't know about forums, but there are websites that list Kodi addons across many repos, and these are often a good way to find out how to get addon X.
