I think you're best off going to the womens' community. Lemmy isn't large enough to have that kind of niche community active. I don't think. But your question will be well received there. I'll try find the community

https://piefed.blahaj.zone/c/womensstuff

This is it, I can't recall how to properly Lemmy link it.

Something like

!womensstuff@piefed.blahaj.zone

Oh yep that's how you do it

Not on Lemmy but I have found Bratabase to be a good resource. Really good measuring guides and pictures of different people in different sizes and bra models.

In general when I'm searching for a particular community, I check https://lemmyverse.net/communities.

At quick glance I didn't see an equivalent of ABraThatFits here on Lemmy.

There does seem to be a lingerie community on lemmy.world but reading the community description, it seems more targeted at pictures of women in lingerie instead of advice on the topic.

Maybe, one of the kink communities could be helpful