It’s hard to find, so far, and expensive, but also required by the latest e-tube app from Shimano, unless you want to use the app on a phone/tablet.
The older app version is still available on their e-tube website, but it apparently doesn’t have all the functionality either. All new versions of the e-tube app are going to require the SM-PCE02 box to access the Di2 system. And they STILL don’t have a mac version of the app. You either use Windows, and download the older app if you have the old interface box, or get the new app and box, or have limited functions with the phone/tablet app. surprise
As long as the older versions are available, use the older ‘charger’ device: Di2 FAQ: - E-Tube Project Professional won't work with my charger. I don't own SM-PCE02!