Blad is right. You can't beat a professional fitting. Because body shapes and types are so varied, there are a million styles of bras and for a male, it makes it even tougher since our chests are more rounded and larger in shape than a female. With more and more men wearing bras due to gynecomastia, bra fittings for men are becoming more common. That is not to say that lingerie stores see a male client everyday, but we are more frequent than we use to be. Start out at a locally owned shop. Being smaller, and local, they will not spread your "business" around like some 20 something at VS or other chain stores. Usually you can go on line and their websites will give an indication if you will be welcomed or not. When you find a shop, call and ask to speak to the owner or manager. Tell her, it's usually a woman, you have gynecomastia and you would like to book a private appointment for a fitting. At this point she will set the appointment up or will tell you they don't serve men. She will probably ask you some questions like do you wear a bra now and if so what size and what style. This is to get some info ahead of time so when you arrive for your appointment, she is familiar with what you might be interested in. Just remember that not only is she trying to make your experience a good one, she also has to be sure that her female clients are not upset by having a male in the store trying on bras, hence the private appointment, usually either just before opening or just at closing so no one else is in the shop. And for a first time, a private appointment will make your more comfortable. And listen to her advice. If she is worth her salt, she will not put you into a bra that will make you look silly or stupid. She will put you in a bra that will work for and with you. After my first fitting, I was surprised at how much confidence I had about my chest in which I had fretted over for years before.
An underbust of 39 and a bust of 42 would be around a 38C without knowing your breast type. Each inch above the underbust number equals one cup size. 36 underbust and a 38 bust is a 36B, 40D is an underbust of 40 and a bust of 44. A good starting point would be to take 6 measurements and use the calculator on Reddit's subreddit, A Bra That Fits. The 6 measurements are: Loose Underbust, Snug Underbust, Tight Underbust, Standing Bust, Leaning Bust and Lying Bust. Plug those into
https://www.abrathatfits.org/calculator.php and that is a really good starting point. Most men are wideset and shallow, meaning clevage is usually not going to happen since the breast sit far apart (wideset) and most of the tissue is below the nipple (shallow). Those type breasts usually work well with plunge and balconette type bras, depending on how close the breasts sit. Wideset breasts usually work better with wide gore (the bridge between the cups) bras in underwire types with the gore lying flat against the sternum and each cup individually supporting each breast for best support. "Soft cups" or non wired bra gores will not sit on the sternum but will float above it, depending on the style. A semi-uniboob effect is what you get with soft cup bras. Again, it depends on your body shape and breast shape and position on your chest. For me personally, I find that a wideset underwire plunge style bra with side support is the most supportive and comfortable bra. Because of my size, my breasts tend to be more on the feminine side, droopy and round where my arms will brush up against the sides of my breast so the side support will pull them in more towards the center of my chest to allow my arms by. Also, the lift of the bra lifts up my breasts off my chest and it is much more comfortable and keeps the breasts contained, or not moving around on my chest. The placement of the wire on the body is the key. The wire needs to sit under the armpits to completely encompass ALL the breast tissue that extends under the armpit (who knew breast tissue went that far back?). If the cup is too small, the wire hits before all the tissue is contained and the wire pokes into the soft tissue and over time leaves marks where the wire is digging in. You want that wire to be "sprung" just a little by the tension of the band to keep the tissue seated in the cup. That is why the "swoop and scoop" method of putting on a bra is so important for fit and comfort. It slightly pulls the wire back as the tissue is put into the cups for a good fit and support. Also you want the band to lie horizontal around the ribs, not diagonally or raised up the back, under the shoulder blades is best, if your breast placement will allow and to be tight enough to not move under normal movement (to be "anchored"), but not so tight it is uncomfortable. For me it took some time to find that sweat spot, but when you do, bra wearing is a complete game changer in comfort and support.
Cup size is not the same from size to size. A 36C cup is not the same volume as a 42C. It is a ratio of cup volume verses band size. This is where sister sizing comes in. A 38C is the same cup size as a 34D or 40B. As you increase in band size, you decrease in cup size to maintain volume and vis-a-versa. In reality, cup size is not a good way to gauge true size, even though everyone does. You can be a 40C with wideset shallow breast type and your projection is not as much as a 40C narrowset full breast type. That is why there are so many different bra types and styles and why a fitter can be your best asset. If I had not gone to a fitter, I would have never known all this and before I went, my bras did not fit and were uncomfortable to wear for long periods. I had self measured myself at a 42C with just two measurements, underbust and bust, which put the band too large and never anchored on my chest and the bra style was wrong for my breast type and the breasts moved all around since the bra wasn't really supporting with up and down movement and the cups were too small and tissue was being squeezed out under and behind the cups and I was being poked by the underwire right into the tender sides of my breast and now after the fitting, I wear a 38DD or 36DDD, depending on the bra and am very comfortable all day, where before after about 4 or 5 hours I was ready to get that thing off! I completely get it why women can't wait to get home and rip their bra off. I was doing the same thing myself. After I got fitted and started wearing the right bras for my body, I go 10 to 14 hours a day in a bra and am comfortable the whole time. That is what a fitting will do for you.