Buying armor is like buying a suit, and all the answers are kind of similar.

You can get a cheap mass produced one, but it won't fit as well as a tailored one, and that won't be as good as a custom made one. You can wear someone else's (within reason), as long as they're mostly the same size, but it won't be great.

A gambeson or arming coat (typo before) is similar, off the rack is fine, but it'll fit much better if you have it custom made. And like bras, being a woman is expensive here. I bought a good one and modified it myself.

I got my armor custom made by a guy in Ukraine like 15 years ago, the whole thing was 6000 euros which was a huge bargain. But if you just want a breastplate for occasional use (larp, cosplay, etc) getting one online and accepting it's kinda awkward is very much an option. If you need it to actually stop swords (reenactment, HEMA) that's still an option, but you'll need to be much more selective.