Well, is the IP address they use changing a lot or is it fairly static?

If it is more or less static you can expect they are propably using it from some house where they have a wlan AP and t is connected to either a wireless or wired broadband.

You can get the provider by digging around a bit, and if it is a wired connection you may be able to sweet-talk the operator into giving you street address, after all that's not classified info.

If it's a wireless broadband, you might not get the subscribes address so easily.