ModMii downloads the hackmii installer and the files required to launch bootmii from bootmii.org. So if bootmii.org goes down, ModMii won't be able to download those files.
Just wait for bootmii.org to be fixed and try using ModMii again to download/build config files for bootmii. Or, if u desperately can't wait, u can try to find some1 willing to illegally send u those files (it's illegal to redistribute those files, but downloading them from the original source is ok).
If you know someone else with a Wii, u can always run the hackmii installer to "prepare an SD Card" in order to get the files required to start bootmii.
Omg do you know how much money I would spend if that was a thing? Especially if you get to assemble the gundams somewhat like those old model toy kits. The answer is zero dollars. I would spend zero dollars. It would still be so good, though.