I made my own ring tone the other day because I recorded something I wanted to use. It's possible. Monstrous pain, but possible. You have to convert it to AAC, then import it to iTunes then sync it over. It was about the same effort of getting a ringtone on my feature phone in the early/mid 2000s. That seems insane to me these days.

You can definitely buy ringtones, you just have to remember that the iTunes music store exists first. I haven't bought anything new on there in over a decade, so I'm waking up to music from High School still.