I'm not sure whether it is generally known, but if you have a hankering to hear South African Boeremusiek played on the concertina, then this website of Kalie de Jager is full of CDs of that music that can be purchased from him. Have a look at the catalog: the number and variety of them are always surprising. I thought the general forum crowd should be aware.
I'm not sure how easy it will be dealing with purchases from South Africa, but send Kalie a note if you are interested. I don't know of any other general source for them outside of that country.
Cheers,
Dan
CDS: http://kalie.boeremusiek.net/cds.html
General information on Kalie's site: http://kalie.boeremusiek.net/