I doubt, even if you found someone reputable selling seeds for 'Blue Satin' Rose of Sharon, you would get what you want. I doubt you can find someone with a decent reputation even selling seed. Even if this seed really is from 'Blue Satin', the seedlings will not look like their parents. They have to be grown from cuttings in order to look like the parent plant.
I did find a couple on-line places selling plants but, as you have a plant, you can start your own cuttings. Until you figure out why your Rose of Sharon is dying, I wouldn't plant another one though.