R.kerstenii kerstenii occurs along the cost from northeast Tanzania to Kenya, they are found in the inner regions (most dry regions) of Kenya and Tanzania too. The subspecies R.k.robecchii appears in Somalia, Kenya and Ethiopia, but as far as I know they were never kept in captivity.
The nominat form was kept more or less often in captivity some years ago but the last 2-3 years the imports of them became rarer and rarer - but I don't know why.