Can anyone recommend additional zither music resources available on the Internet?
Well, it's not zither-specific, but I enjoy using TuneIn Radio
for listening to radio from all over the place. It's sort of a one-stop-shop that collects streaming radio and internet-only streams from various stations. They have the radio station your friend sent over (BR Heimat), plus plenty of other "volksmusik" stations.
You can, theoretically, just go to the websites of most these stations and listen that way, but it's nice to have it all in one place. You can keep a list of favorites, check out recommendations, etc.
I was using it just last night, using their smartphone app (with my phone plugged into the kitchen radio!), to listen to Alpenradio Volksmusik
, which based out of a small town in Bavaria. They did play a zither tune now and then, as I recall.
In fact, I just popped over to check and they're playing "Der Weg zum Herzen" by Egon Finazzer right now. Sounds like a zither/guitar duo.