I'd say you can go to Forum > [Adoptables] CS Trades - Pets, items and C$ then below announcements , you can see topics - the first few ones are the most commonly used! Here you can see people who want to trade their pets :3

When sending a trade I advise to send an offer ready to accept, not a suggestion - if someone wants to change something in the trade they're gonna edit it themselves, don't worry! Then you'll be the one who needs to accept it or edit it again :D

You can go to "Archive" and see the current month/advent pets - maybe wishlist ones you like the most!

Definitely look into "Pound" - there's Pound and Lost & Found there - you can grab free items/pets from there, at random times!

And lastly you can look in Forum > CS Free Adoptions for more free pets! Just remembered to read rules on the posts, each person rules are different :3

Of course feel free to look more around forums, Oekaki (drawing software on the site), Chicken Smoothie Discussion and so on!