Lolas! (shame on me; I keep leaving out the exclamation point) have just come out with unrigged (and hence editable) mesh breasts; they are called "Tango". Initial photos look very good, though of course the proof of the pudding is how they look in-world, and reviews I've seen are quite positive.
I see that people are already working on appliers, so I hope that Tango users will have a broad choice of clothing soon. (As Hugsie points out, it may be possible to apply a clothing layer texture and then tweak manually.)
Googling turns up a goodly number of posts about them; Colleen Criss's very enthusiastic article has nude and clothed photos and links to other articles, and Hugsie has details on applying skin textures to Tango breasts.