They're called breast forms, and yes, they do exist

But also you don't need anything especially fancy, I knew a girl who just put socks in her bra haha

The term you are looking for is breast forms

On my partner's recommendation, I got some "Bra Buddies" from Unclockable. They're just meant to give a cup-size or two boost to whatever you may have, and not as full-figured/over-the-top as a traditional breast form. I think they're great, but I also deeply regret getting them because they were a little overpriced and everything else I got off of Unclockable was actively garbage. But if you've got the money to spend, I guess.

I'd recommend looking for bra pad inserts. A lot of bras are natively designed for inserts to begin with, so you can find ones that are thicker/stiffer to give a bigger and more feminizing look without breaking the bank. They're not going to feel as supple as a real breast (or a Bra Buddy, to be honest), but neither would a 3D printed PLA boob.

Socks and breast forms are great ideas, and I want to add Knitted Knockers! They were originally designed to be for cis women who lost their breasts due to cancer and needed accessible and comfortable gender affirming prosthetics. They aren't complicated to make, most of the time the pattern is free, and some places even have free distribution programs!

I got gifted a pair of bra pads from a friends mom that i could just put inbetween the fabric and just have a pretty bra that i could wear pretty much all the time :3