>>"One seller said perhaps 芙蓉茶 is a (different) kind of green tea and that perhaps what I meant was 芙蓉花 and that these kinds of flowers are grown in the villages, but not sold as tea."
Yes, the seller gave you part of the answer. What you are after is not tea at all.
Tea is made from a specific shrub or tree (camellia sinensis.) It can be green tea, red tea, Pu'er tea, Oolong tea, and so on, mainly according to how it is processed (oversimplification.)
What you are seeking is flowers from which to make a tisane or herbal infusion.
Therefore you need to be asking for 芙蓉花 and to be explaining that you want to use them to make 芙蓉茶。 The word 茶 is used in a loose sense here, not in a strict sense.
I had thought hibiscus was readily available in supermarkets here, but since telling you that some days ago I've had a closer look. Seems I was mistaken. Have not seen it when shopping.
玫瑰花 and 菊花 (rose and chrysanthemum) are abundant and cheap. But I didn't see hibiscus.
A next logical step would be to try some TCM pharmacies. They are all over town, in every neighborhood, often clustered near a hospital.
They stock all sorts of unusual herbs, roots and bark from which people brew infusions for medicinal purposes.
They might have it, or they might not. It could just not be available in this part of China. (I really don't know.)