From a ciswoman with an unusual size (32H):
1. Everyone has to try bras on to purchase. Even ciswomen. Welcome to being female! (and yes, it can be annoying.) I hear menswear really does work in terms of literal measurements. Womenswear is often devoid of logic. The band size is usually somewhat reliable though.
2. Victoria's secret is useful for being mainstream, having an online catalogue+ myriad stores to work with, and if they still operate the way they used to, online coupons (I haven't used them in say, a decade? I sized out of them.)
-but be aware last time I tried them, it seems they do vanity sizing- at one point I knew my band size and theirs was bigger than it should have been.
-some of them have done some very lazy fittings when I've been in there. I've also seen super lazy fittings in other, pricier stores, alas. (ie, clerks who just wanna shove you into a bra and get it over with, don't do the fitting properly.)
-A friend of mine recommended aerie- not sure of their size ranges because I don't fit them either, and they seem to cater more to the teen/ youngish population, but pricing should theoretically be reasonable.
3. Some places like figleaves will let you purchase it online and return it, although they are based in the UK so it ships from there. They also have a wider range of sizes than VS does, even if their panties were terribly boring last I checked. VS does have cute cotton panties (not using cotton in the crotch is asking for a yeast infection), even if they aren't as good as they used to be.
4. Aside from the literal measurements, I would look at the advice online that shows you what a bra that properly fits looks like. At one point figleaves had a good tutorial, not sure if it's still there. Check for pictures- things like the center of the bra (near sternum) should be flush against the chest with most bras, where the band size should lie, which hook you should fit when first purchased to allow room for fabric stretching, etc.
