Not sold at retail, but you can browse SmallRig
Some links here are affiliate links. If you buy through one, Creator Gear earns a commission, at no extra cost to you.
Creators cited with this
Jason VongYouTube, 675K followers“The smallrig thumb grip and the pinky extension plate, I believe, are sold separately, but these are officially partnered and made with Panasonic, so it does blend in seamlessly, and it makes it feel really good to hold.”
Watch the video at 8:16
Creators who use this also use

SmallRig thumb grip
also used by 1 of them
Sony gray body/lens caps
also used by 1 of them

Freewell x Brandon Li 82mm Variable ND/CPL Basic Kit
also used by 1 of them

Lenskirt Anti-Reflection Lens Hood
also used by 1 of them

RØDE Streamer X Audio Interface and Capture Card
also used by 1 of them

RØDE X Streamer X Audio Interface
also used by 1 of them