Well... That's a custom made tank. Similar to the Triton tanks on yore. There is nobody I know of manufaturing a similar tank for small change. Its going to cost ya. You can however find a tank with a similar shape, a ball peen hammer, and make something similar yourself. Or you can try to make a fibreglass one, but its no small task and glass tanks arent the best option for a street bike all the time.
RocCity can make you that tank from aluminum or steel, but again it won't be cheap. Its extremely labor intensive and a real art. Which again, isn't cheap...