Hello! Zedge is a great app for this. You can download it from the play store. Once you find a ringtone you want to use, click download and it will be in your system sounds when you want to find it again.
I make my own ring tones by downloading the wavepad app then recording the sound or tone I want, adjust the volume, speed and length then use the file manager to copy it to ring tones or default message tone