ebay would probably be your best bet. I can't remember seeing them advertised by any of the distributors. Possibly try to find some of the Nazi surplus ammo, and just remember to pick them up when you finish shooting. they drop out of the bottom of the rifle after the last round is chambered.
This link from a google search: https://www.buymilsurp.com/magazines...s-c-24_27.html
WOW!!! $10 each. think I'm gonna do some "unpackaging", and ebay listing tonight!!! I've seen the ammo, two clips of 5 rnds, for $8.00-$10.00 at gun shows. I tried to make a deal with them for a trade on other ammo for some of the 3 or 4 hundred rounds I have left. they said they didn't want any more.