The Scottish Straight is a Scottish Fold without the folded ears as you might expect. These cats have to be created by breeders because they cannot mate Scottish Fold with Scottish Fold because the cat carries a double strength genetic mutation which causes the folded ears and when this happens there are abnormalities in the cat such as vertebrae being fused together and cartilage growing on the feet which causes lameness et cetera. So they have to create Scottish Straight cats as part of the breeding process which is why you will see them for sale. It is an unfortunate consequence of creating this cat.
These cats are created when out-crossing to a non-Scottish Fold when, as I understand it, half the litter has standard ears.