When my run flats ran flat I had to cut the 'run flat' insert to remove it from within the tyre.
The insert was, if my recollection is correct, made in France but I cannot remember the name on it.
I then used a tube, having first inserted the rim liner to protect the tube from the split in the split rim.
I have had no problems since and have, at the moment, 2 run flats and 2 tubed.
So the tubes are OK inside the tubeless tyres (so far).
N.O.S. thanks for the contacts......will try them. Not so sure about trying 12.00R20 although they are more readily available (new from Vass).