I've got rpcs3 setup on my steam machine but I've been out of the piracy game for years.

Edit : you all are amazing thank you.

I would try fmhy .net

if you're on windows, you can try searching with qbittorrent's built in search

You can also find a lot of stuff in the Internet Archive, though download speeds are slooooooooow - https://archive.org/details/ps3-games-part-2

Vimm's lair has a bunch of games, too, but it limits downloads to 1 at a time, they're all self hosted.

You can also search for sites on Yandex.com, they still show pirate sites. Be sure to have uBlock Origin on your browser

vimm.net

Rip myrient Minerva archive has replaced it if you have a good VPN like mullvad and also qbittorent

I've never emulated PS3, but FMHY (freemediaheckyeah) is always my go-to starting point for answering this kind of question. They usually have awesome recommendations:

https://fmhy.net/gaming#playstation-roms

For me, private trackers, specifically TorrentLeech.

There's a great utility that I've used to get games as well.
Nopaystation is the place that lets you download the games from the store, not the iso files.

There are very few games that are exclusive to the PS3 now that unofficial PC ports are a thing. Games like God of War III, the Uncharted trilogy, their PS4 versions can all be played directly on PC. So I suggest you check out if the game is available on PS4 before you deal with rpcs3. Even if it doesn't have a port, it might be better to play the PS4 version on shadPS4 than playing it on rpcs3.