SDL2_mixer soundfont to play midi

I’m using SDL2 and SDL2_mixer in Linux. The following errors show up when loading and playing midi files :

fluidsynth: error: Unable to open file “/usr/share/sounds/sf3/FluidR3Mono_GM.sf3”
fluidsynth: error: Couldn’t load soundfont file
fluidsynth: error: Failed to load SoundFont “/usr/share/sounds/sf3/FluidR3Mono_GM.sf3”
fluidsynth: error: Unable to open file “/usr/share/sounds/sf2/FluidR3_GM.sf2”
fluidsynth: error: Couldn’t load soundfont file
fluidsynth: error: Failed to load SoundFont “/usr/share/sounds/sf2/FluidR3_GM.sf2”

Where do we find the appropriate soundfonts ?
How can I configure SDL2_mixer to use other soundfonts ?