
Alphagvrd now offers camera skins for Ricoh GR III cameras (currently on sale: $14 off). Several different colors are available: There are also some GR III camera skins sold on eBay. More Ricoh GR accessories can be found here.
You can now buy optical waist-level viewfinders for Ricoh GR cameras on Ebay and Aliexpress. The viewfinders have different frame lines (28mm/35mm/40mm/50mm), so make sure you pick the correct version (two different models are available). More Ricoh GR accessories can be found here. Additional pictures: