No, the soundfonts can be anything as long as they are in made in the standard order, it doesn't have to be from Pokemon, or from any game at all in that case
I don't have a specific site I go to, but you can search things like "nds soundfonts" or "snes soundfonts" or "high quality soundfonts" etc and find quite a few nice results
Also, making soundfonts is a long tedious process, you need to sample hundreds of instruments in order to do so. MIDIs are actually just basically a text file that uses a soundfont in order to make noise, without a soundfont midis don't do anything, and I don't believe MP3 to soundfont is possible