I am not promoting piracy, every ROM that I have is for a game that I have a physical copy of. To find your own I can only point you to google. Obligatory “I am not responsible for anything that happens to your switch, blah blah blah…”.
To Install NSPs you will need your system keys, I discuss this in the Kosmos Video linked below. Go to https://switchtools.sshnuke.net and download the newest version of “biskeydump” (v7 as of writing this) and inject the “biskeydump.bin” payload using the method of your choice and select the dump keys option. You need to run this payload FIRST to dump keys that the homebrew application needs to get the rest.