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