In my quest to find clothing that fits a slim/small/flat-chested profile, in January 2016 I came across a store called Mikunch. I've found that Japanese stores in particular are more likely to cater to small-breasted females, whereas an awful lot of Second Life starts out at pretty large and scales up from there. Mikunch is just such a place -- a small store with cute clothing, mostly dresses, that work very well for small chested girls or even possibly flat-chested girly bois.
This was also one of those stores that has moved, so the first attempt to find them ported me in to an empty lot. In these situations I look up the creator and check their profile picks, and that led me to the actual store. I bought several dresses that suited my desire for a look with no obvious breasts, or very small ones.
You'll have to forgive me, I tend to take my pictures in scenic spots with my character off to one side and I use these for computer wallpaper. ^_^ I forget what sim this was in since I did this early January, but this outfit is the Mikunch Embroidery min OP, sweater layered. 200 Lindens (for the dress and sweater).
This outfit is the spring onepiece with puff sleeves, 300 Lindens. They have several colors/patterns for each of these designs btw.
This one is the draped OP w/pocket-T (white). 350 Lindens. This one comes with a HUD so you can have multiple looks (colors and patterns).
Some of these show small breasts, and all of them are ultra-girly, but I think they can work for a femboi as well. At least, one who is as girly as mine. ^_^
This is an outfit using a Mikunch tee shirt (2014 summer-T). 250 Lindens, this also comes with a texture-changing HUD so it's more like 10 different tee shirts in one box.
There's more at the store. An awful lot of it is multiple textures with a HUD so that you're getting several outfits for 250 to 350 Lindens. Very good deals. ^_^
