I have been collecting free asian heightmaps in prep for my x-carve, here's the collection so far, all freely found on websites, not behind any kind of paywall or adwall.
I really prefer heightmaps to STLs, as you can load and layer them in different carving apps. I guess the more depth you get you would want more dynamic range in your grey values, also, I often see JPEG artifacts in the displacement, but what do you want; they're free and you can smooth or sand that off easily
I hope this saves you time, it took me a long time and help of a friend who speaks chinese to navigate the websites.
And if anyone has non-asian heightmaps, like Roman or historic or pub domain, feel free to share.