This is for people using Captive Portal and when you want users to be able to change their passwords.
The cheapest solution we found was by purchasing Radius Manager: http://www.dmasoftlab.com
It's about $100 to start and $50 per year after that.
It provides a web browser interface for users to change their passwords.
You'll have to add a server and install linux. If you give dmasoftlab the public static IP of your linux server, they will remotely install and configure the software for you (after you do a little prep such as enabling SSH).