eBay or Amazon Japan (bluetooth or 2.4Ghz) would be a good starting place.
But, be forwarded of potential issue(s) with Windows.
I’m tempted to to buy one of those keyboards…but it may not actually be that much more efficient (past the learning curve). For example, by the looks of it, it’s two strokes to type any letter that uses han/dakuten which is the same for typing the letter using romaji. Also, notice the letters up there with the number keys which adds an interesting dynamic.
Anyone have any experience using a jp keyboard on an English system? Is it an overall plus?