As anyone who has ever searched online for lingerie made for trans or cross-gender people will know, options are unfortunately quite limited. It takes some exploration and experimentation to find out, first of all, what works best on your body and for your personal sense of style, and second, which products will meet these needs.
If you're a trans woman, you may have measurements that fall outside standard sizing, especially when it comes to bras. That ends up drastically limiting what is available for you to choose from, but I'd like to offer a couple of suggestions for finding a bra that fits both your size and your style.
In her recent post on lingerie and androgyny, Rose of TLA points out that "pieces that would be good fits for nonbinary people often get sold as 'men’s fetishwear.'" When sold as fetishwear, lingerie tends to be sexy/exotic and super-femme, which obviously doesn't appeal to all (or even most) trans women.
The Flatter Me bra (pictured above), by designer Jane Hodgdon, comes in smaller cup sizes and larger bands -- from 40AA to 46B. I discovered Jane's bras when I was looking for AA and A-cup options for myself, and I was very impressed with them. The Flatter Me has shallow cups for a good fit without gapping, lightweight underwires, and removable padding (fairly modest and subtle; even if you use the pads, they don't give the appearance of a much larger bust). Also noteworthy is the sleek, no-fuss look of this bra-- perhaps a welcome change for those seeking an alternative to fetishwear.
For band sizes 38 or smaller, check out Lula Lu. This brand is a reliable source of well-made bras with or without underwires, in AA and A cups. They offer everyday t-shirt bras similar to the Flatter Me, plus a selection of lace styles if you're looking for something fancier. Although Lula Lu is a petite line, the bras are also a good fit for small-busted women (cis-gender or trans) with average figures, meaning they work well on wide-set breasts.
If you're not sure what size you need, the transgender bra sizing guide at A Bra That Fits on Reddit may be helpful.