Not advertised, you have to look for them.

I actually found it on spirit.com, you go through the check out process and you can see the fare details. We were going to spend over $70 in taxes if we just listed as non-rev, so I figured we might as well be confirmed. I bought the ticket in FLL and avoided online fees.

You can also ask the gate agents to look up fares if they have time, and they can quickly tell you if there are any penny fares.