Raven Ivanova sells these avatar attachments at her store Smashing Prims, where apparently they're quite popular -- to the tune of about $2,000 after only 2 months on the market. Residents can buy the breasts as separate attachments, or as adjuncts to dresses that Ivanova also sells at her store. For the full story, and a video of the flexiboobs in action, check out Hamlet Au's New World Notes.
I'm going to have to stop predicting the future. Or failing that, predict something useful to me.