I like the single hollyhocks. The bees prefer them too...
Single hollyhocks are not stocked in any garden centres anywhere near me.
I've grown all mine from seed from T&M. They offer a couple of different varieties of singles - both very pale, and mixed strong colours. They are easy to germinate, and seed sown now should have produced flowering plants by this time next year. (T&M also supply single varieties as plug plants).
I've never tried root cuttings. If you can, cut off a small piece of root with a sharp kitchen knife, pot it up, and give it some TLC.