1 - A good number of addons can be found on the launcher already. Click Addons at the top before launching the game. This is an easy way for new players to get the addons for the right game version.
2 - Some of the authors in this forum keep a copy of their repos on the OctoWoW Git website -
https://octowow.st/git/explore/repos
3 - Google searches for "OctoWoW Addons" or "TurtleWow Addons" you might find more niche addons for this version of WoW not found on the launcher, but many are not maintained anymore but may still function.
Definitely start with the launcher. Even if you don't use the launcher, it has links directly to the addon's Github repository where you can manually download and install the addon if you choose.