So, I was going to be all organized and tour the Mammatus Mall and Northstar Mall and the other malls where one can find prim breasts and clothes for prim breasts, gathering data... and then talented and drop-dead gorgeous Mesha Sewell IMed me about Nanami.
Nanami makes clothing that works with Implant Nation, eCorp, and iBoob prim breasts. (I should mention that earlier Implant Nation prim breasts don't support third-party clothing; here and henceforth, when I say a top works with Implant Nation prim breasts, I mean the later versions.) The styles vary from a demure white Victorian dress with matching hat to more modern and more daring outfits. They're beautifully made. I'm sure I will buy that Victorian gown someday.
There's clothing for guys there, too.
The main store is at Saphir 59, 180, 22. Since it's called the main store, I have to think there are other branch stores, but I have yet to find one. I'll give pointers as I find them.
UPDATE: Nanami does indeed have various branch stores; one is at the Mammatus Mall, up at the northeast corner of the third floor. (Teleports into Mammatus always take you to a spot from which you can teleport to one of several areas on the island; choose the Mall, and once you're teleported there, you can get to the store.))