The r/abrathatfits subreddit, which discusses proper bra fitting. It's reddit, so I'm not surprised, but it's a huge shame because their measurement and fitting advice was without exaggeration nothing short of life-changing when I learned it. Tl;dr:
bra fitting in most normie stores (like Victoria's Secret, Aerie, etc.) is completely arbitrary and pants on head retarded, the popular conception that A-D cup sizes are normal whereas DD+ is big (with little focus on band sizes) is completely false, and most brands carry a narrow range of sizes that don't fit a lot of normal healthy women, especially with a lack of sub-32 band sizes, even though it's very common.
Unfortunately, the sub's slogan is "Anyone who wants one deserves a bra that fits"—and they mean ANYONE. You don't even need to claim trans identity, although you'll get tons of hugboxing on how stunning & bwave you are if you do, so expect tons of attention-seeking and unsolicited interjections from that crowd. No, not only can you claim to be a man (which, ok, some men have gynocomastia or whatever, fine), you can literally say "I'm a fetishist who gets off to this

" and they'll ban the women who complain. Maleposters of all stripes are also extremely likely to skip lurking and make low quality posts not providing any relevant info (besides blogposting about being male) but expecting to be spoonfed. It's like the equivalent of going into a PC building group and saying "hey guys im epic gamer who wants super cool gaming PC what should i buy" with no mention or knowledge of specs, budget, etc.
There's lame hugboxing around women claiming to be men or non-binary too but they're usually less obnoxious about it. Expect not to be able to talk about bras as a woman-centered topic however.
First screenshot was a pinned post for a long time, I know the other two aren't the best examples since they're old but they were the first examples I grabbed and that type of post was common when I was still checking the group. Also, it's known that they have a huge problem with creep DMs, which the SPOILER tag on every post is supposed to somehow help deter. Why it's somehow ok if a creep adds "btw i want a bra haha" is beyond me.


Still, I can't understate how helpful their information was, so if you stick to the calculator and wiki posts, it's still a good resource... just hard to recommend.