Its not a breeder or seedbank, clone only strains are only available as clones. Mostly they are special selected phenotypes of a commercial strain, or old classics who was keeped by someone somewhere...

A clone only strains floating around the California Medical Scene, SoCal Master Kush is said to be a selection from master kush genetics and is the staple of some of the world's most potent strains. It's extrmely frosty and potent for a Master Kush with more of a moderate yield than can be expected for most Master Kushes.

Genetics: Master Kush Selection

This cut of Master Kush is said to be the kush used to create another clone-only, Bubba Kush, as there are more than a few similarities between the two.