Heavystar seems to have faded from the auction scene. It’s unfortunate.. they were a useful source. The salient point is the 40mm filters must have ‘fine’ thread (0.5mm pitch) instead of the usual filter thread which is .75mm pitch. The Fuji x10, x20, x30 series uses this same 40mm fine thread. What you’re trying to find is an adapter with 40mm fine on the lens-side and some ‘standard’ filter thread (43mm, 49mm, etc) on the outside end. I have S&W step rings of this type.
Unless you already have 40.5mm filters. Those are also fine thread, and a 40-to-40.5 step ring will be fine pitch on both sides. If you can find one.