The ringtones are in:
They are .ogg files.
You need a file manager to manipulate them.
es file explorer is my favorite. Any ogg file you stick in there will show up in the phones ringtone menu. Never tried sticking mp3 files on there, if that's what you have planned, post back if it works.
You know you don't have to put your custom ones in there just to use them... Just navigate to where you keep them in your music player, or whatever, And long press to set as your ringtone.
Apologies if you already knew that... Just sayin'!