The only PS addons I actually use for PS are Topaz Clean and Denoise for cleaning manga scans and Sergei's Brusherator and Perspective Tools addon. I also use the Apply All Adjustment layers script from Artstation (free) and a delete all empty layers script. PS has fugloads of features (some things I've never used, I used GIMP for like 3 years and PS for a decade or something) so I haven't found addons to be necessary yet.
On /ic/ pretty much everyone is a total normalfag and is also clueless and haven't gone on a real dive into a bunch of programs yet, so... And PS is like one of the few which can potentially get addons.
Cgpeers (private torrent site on perma freeleech) is a pretty gigantic must for torrenting everything art related. /ic/ can only be relied on for artbooks. It used to be open on two days of the month, but not anymore because people uploaded malware so they closed registration. The only way to get an invite at the moment is through myanonamouse and four weeks for power user 'uploaded' 25 gb (there's bonus points so you don't need to upload actual data) or wait 6 months on a regular account.
However, cgpeers won't have everything. And most of the time if you want stuff like Lazy Nezumi you'll have to pay because it's drm'd to the moon and the back. Artists are as poor as dirt, most of the people making addons/tutorials etc are as poor as dirt, pirates are as poor as dirt, artists don't generally know any programming and there's pretty much only one scene group that cracks programs and they are very very unreliable. The deeper you go, especially if you go into 3d, the more you'll find programs that essentially cost between $800-$2000 for a license or which are subscription only and the only cracked version was uploaded like 10 years ago and all the alternative programs have twenty dozen trojans embedded in them or they're cracked and they crash every hour because of anti piracy.