I feel your pain, Sister. I struggled with bra sizing for years. I'm not expert but happy to share what I know.
Once again, sizing depends on brand. Most current major brands have or are moving toward the band size = snug underbust size. Personally, tend prefer Bali, and find the sizing to be pretty consistent across the underwire bras in their line making it easy to buy their bras. I find them well made, reasonably priced and comfortable. In this post I'm going to use examples of Bali bras I am familiar with to illustrate points--not a promotion of this brand.
Many older bras are sized by the underbust + 4 band size. I wonder if this is why your current 42 band feels about right. I have noticed that the cheap Chinese brands on Ebay size the bands on the order of the underbust + 4 sizing. I'd stay away from that stuff, go with a few decent major brand bras that actually fit properly.
You can look at my thread on this site on silicone enhancers, pads, "chicken fillets" or "chicken cutlets" as they are variously called. Figure out the bust size that looks about typically right on your frame size, usually about a B-C cup for given band size; or the whatever size you are inclined toward (buxom?). Try out the added volume using the short grained rice in a piece of a stocking or pantyhose leg trick, being careful to measure the volume of rice accurately. You can do this repetitively at different volume to do a test drive of different bust sizes before plopping down money on some decent quality silicone gel enhancers that will make up the difference between your actual breast volume and the volume of the 38B or 40B that is probably what you are going to end up wearing. I am partial to the Softleaves line of enhancers Again, my personal preference based on having tried many, not a promotion), but there are numerous others such as Braza and ones you can buy in the major department stores. I'd advise against foam devices as they do not mimic the texture, bounce, density and softness of actual breast tissue. With the ones I have on right now, I can only tell they are not me by virtue of the absence of sensation of touch over the silicone. I would hazard a guess that if anyone were to touch my chest, even while wear in a thin unpadded bra with these in place it would not be possible to tell it is not all me.
This gets to the next questions: wire or wirefree, padded or unpadded. The molded lightly padded underwire bras are a godsend for most of us as they help provide smooth, natural, attractive breast shape. For example, look at the Bali 3470 or B534 both available in 38, 40, 42 B and larger cup sizes. Of note, repeating an earlier post, Leading Lady underwire bras come in A cups to huge band sizes which I am sure was part of the business plan to accommodate larger women with small cup size but also the TG customer. I'd love to know what proportion of the >38 band A cup bras in their line are purchased by genetically male TG buyers. There are a myriad of other brands--enjoy the shopping...
I am of the opinion that the underwires are our best friends since they help contain the breast tissue within the cups. Be absolutely sure to "swoop and scoop" as I alluded to in the prior post. This is a critical step, moving as much of the tissue into the cups while leaning forward. This is shown in the video and will make the most of what you've got since the typical breast anatomy of a transwoman is tendency toward wide root, shallow breasts.
Even unpadded bras with underwires work better than you'd probably expect but will not provide as much shaping as the molded lightly padded unless you go for ones with highly structured, usually 2-3 seam cups such as my favorites, the current model Bali 6543 "Lace Desires" or the Carnival 509. Both are balconette designs which place the straps toward the outer edges of the cups which is generally a good design for the TG chest and are pretty well structured giving very nice contour and shape. Surprisingly, the Felina Harlow, a fairly daring unpadded highly structured lace balconette, looks pretty good with the help of a minimal mount of silicone. I had one of these but just found it looked like I was trying too hard, bordered on age inappropriate, too sexy.
In the world of wire-free bras, I'd look at sports bras and bralets. These are good for the wide root, shallow anatomy. Sports bras will tend to flatten what you've got but are good for the purpose of support and prevention of nipple friction (such as with exercise) or the "headlights" phenomenon in the cold. Most bralets, particularly the cute ones, often do not come in larger band sizes so are difficult to find. The bralets provide much less support and shaping. There are hybrids that are wire-free, usually seamless knit design such as made by Barely There and several others. This is a good option as you will be able to find a larger band and elastic cups that will more easily accommodate your smaller current breast size and shape. The Barely There (Bali-Hanes) line has some bras which are sized S-M-L-XL-2X which tend to be the ones I've found to have the stretch cups and are easy to fit and wear. There are others in this line that are sized by traditional band/cup sizes which will have more cup structure.
The appliance type, utilitarian bras such as Bali 3820 are just too homely, approaching ugly for my tastes. There are some that are a bit better from other manufacturers but I have yet to find one I really can say I find attractive in a larger band size and I don't tend to like the flatter less rounded separated shape usually resulting from these bras.
Don't forget to visit HerRoom.com. It has a huge inventory and is a great website with lots of customer reviews and recommendations. This is a much better source than Fig Leaves or Bare Necessities. It has a very TG friendly policy, even recommendations for best TG undergarments and lingerie based on men's or transwomen's customer reviews on the site:
http://www.herroom.com/lingerie-for-men.aspxEven better, they are extremely helpful on the phone. Yes, you can call and discuss sizing and fit with a real and to my experience, knowledgeable salesperson--something like a transtelephonic virtual fitting. The salespeople are gracious and don't flinch when you explain that you are a transwoman looking for a particular garment. They have frequent sales, stock most every major brand and have a very liberal return policy. Great company to do business with.
But ultimately, here is nothing like a fitting with a real human being at a store.
Best of luck!
