Asking here probably isn't the best thing to do since this is for older systems. The 3DS is currently supported so you'll have to dig deep for a website to download it from, and with the way piracy goes you're probably gonna have to torrent it.
Not recommending it, though...
"NintenDON'T sue me please haha"
I keep finding sketchy websites to Download Ultra Sun from, is there a safe place I can download them from.