I asked my LBS, and they got a bag of 4 from Specialized. I’ve used 3 so far, and plan on removing the handlebar remote soon and plugging the 4th hole!
They’re just rubber bungs that you push in.
Wondering if the Di2 plugs For there road bikes would work . I’ve asked a few shops and a no go .
Asked the shop I bought my Lego SL from never even given me a email response .