If you go to Futanari Palace (
www.futanaripalace.com) and sign up, search for the artist Linty. (S)he does a lot of large futanari work, but this is really the only bit of her work that explicitly has breast expansion in it. There are links in her signature to her other stuff, and there's a lot of talk of when the next volume of her manga comes out, but no real set date or even speculated time. Apparently (s)he's a well-known artist on those boards, so there are stories and the like involving the self-depicted character as well. Big breasts and large futanari, but like I said, the only thing I've seen of hers involving actual definitive breast expansion is that manga. So if you're a futa fan, you'll definitely enjoy searching for her. Otherwise, stick to the few pages that involve the growth.