I did a bit of a deep dive ~4 years ago because my daughter was having problems finding a bra that fit.
My takeaway: There ISNO "bra size". There are two to three different systems (IIRC a more common one based off "victorian shirt size").
To get a good first approximation, find the bras that tell you how they are sized and give you instructions on measuring. You will probably need these measurements: under bust, bust, over bust, leaning bust. From there, you may need to try "sister sizes", where you go up a band size and down a cup size or vice versa. IOW, if you measure 38C, you might want to try a 36D and a 40B as well.
As other have mentioned, abrathatfits is well regarded.
Yeah, early in our relationship I took my SO to a bra specialty shop, as I was certain she wasn't 80 (36US) in circumference based on my previous partners.
Turned out the right size was 65 (30US), she just hadn't found a large enough cup size at the big chains she'd been shopping at less than 80 (36US).
However, even armed with the appropriate size, we'd still end up testing like 10+ bras when we went shopping for a new one before finding a good one.
Some brands just didn't have any cups that fit her breasts well. They might for example have enough volume per se, but perhaps the top of the cup would "dig in" in midway up her breasts, forming a visible line. Or the cups would be too narrow, so hear breast would "overflow" on the sides. With others there were size variations that lead to poor fit.
My takeaway: There IS NO "bra size". There are two to three different systems (IIRC a more common one based off "victorian shirt size").
To get a good first approximation, find the bras that tell you how they are sized and give you instructions on measuring. You will probably need these measurements: under bust, bust, over bust, leaning bust. From there, you may need to try "sister sizes", where you go up a band size and down a cup size or vice versa. IOW, if you measure 38C, you might want to try a 36D and a 40B as well.
As other have mentioned, abrathatfits is well regarded.