TTS, download as MP3 for end user

I have an idea, but I have no clue how to make it happen, although, I am sure it’s possible. I am looking for a text to speech that a user can use to get their text not only to speak but to also be able to download it as and MP3. For example say the user typed in directions, then the TTS would process the text into speech and ask the user if they wanted to download it as an MP3. Any ideas????