From Amazon
Ricoh 37828 External Mini Finder GV-3 [Compatible models: RICOH GR IIIx] [Optical viewfinder with a 40 mm angle of view attached to the hot shoe] [Field of view approx. 85%]
The product is available from £246.95 at international stores.
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Ricoh GV-3 External Viewfinder
OnBuy
Ricoh GV-3 Mini External Viewfinder