Check out Thebreastformstore.com and call/email Victor for some advice on the best size. They’ve also got size charts on their site, but Victor can help you get the best size and shape. (Not only should your form be proportionate to your size, but if you’ve got large pecs, you may need a form that’s got a curved back in order to fit better.)
It’s better to err on the side of being a little small if you’re interested in going out in public. You can always get a set of bra enhancers if you want an extra boost for certain outfits. I’m normally a C cup, but there’s a couple tops that look better with the bra
filled out a bit more.
BTW, just in case you weren’t aware bra band sizes aren’t your actual chest size, you actually add a couple inches. Shapings.com has the best fit chart I’ve seen and if your actual measurement is 34, you’re probably either a 38 or 40. For females, if you’re an odd number you go up to the next even size, but since our “breasts” aren’t attached to the chest wall, I find it can be a good idea to go down to the next even size, so that the form doesn’t potentially slide under the bottom of the bra. (That’s what happened once when I was dancing.) It just depends on whether you find a smaller size comfortable. One other thing that complicates sizing is the amount of stretch in an individual bra. I got a cheap bra where I needed to go down to a 40 because it stretched so much and when I got a quality bra from Victoria’s Secret, I discovered I needed to go up to 42 and even that’s slightly small.
BTW, definitely also check out Shaping’s bra fitting info. It can be a challenge, but if it’s any consolation most women don’t wear bras that fit properly either. Fortunately, we don’t need to wear our’s full-time, so a bad fit is annoying but not as critical.
One thing to beware of are breastforms that have “mildly erect” nipples, which may show through clothing—something you may or may wnat. Most women are embarassed at having “headlights,”which is why some bras have a light foam liner and I’ve heard some women will stuff a bit of tissue in the bra over the nipples to hide this.As far as the bra, a “pocket bra,” specially design to hold breastforms, will definitely keep the girls in place, but don’t feel constrained to get a pocket bra. My experience is that any good full coverage bra works fine (if you’re out dancing, etc. additional adhesives are good for a little extra security). If you’re shy you can do mail order—that’s how I bought stuff from Victoria’s Secret. At worse some clerk some where might have suspicions, but you’re just a name to them anyway. But I’ve heard from enough other crossdressers who’ve gone in for bra fittings at Victoria’s Secret or Lane Bryant (whose Cacique line is made by the same manufacter as VS) that you’ll rarely be the first crossdresser they’ve encountered and it’s usually no big deal to them. After a sale is a sale.