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You will need to cut off the dead growth. It will likely not recover from a cold snap like that.

The new growth will take over as the new shrub. In fact, pruning to the shrub to the ground is a common rejuvenation tactic for when the shrub fails to grow properly.

Here is an article that will help:

https://www.gardeningknowhow.com/ornamental/shrubs/shgen/rejuvenation-pruning-tips.htm

Here is an article for the care of the shrub:

https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/rose-of-sharon-bush.htm

We do not offer information on where to purchase, only gardening information. I would suggest vendors on Amazon, Etsy, or Ebay. You can even try your local nurseries.

What I can do is offer an article on their care. This will help:

https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/rose-of-sharon-bush.htm

We do not recommend specific vendors to purchase from. I would suggest sourcing some, locally, or checking around with online vendors, such as on Amazon, Ebay, or Etsy.

In the meantime, I can offer one of our articles that will help you to care for the shrub:

https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/rose-of-sharon-bush.htm

It will not be enough to cut them down, unfortunately. They will respond by growing back stronger and larger. You will, also, need to kill off the roots.

this article will help:

https://www.gardeningknowhow.com/ornamental/trees/tgen/how-to-kill-a-tree-killing-trees-in-your-garden.htm

Rose of Sharon is typically hardy in zones 5-10, so you are on the edge of the zones. I would wait till spring to plant it so it has time to get well established before winter. If you mulch real well next fall, it may sail through winter. If the top growth dies, it may come back from the roots.

Also, when shopping, check the plant tags, you may find one that is hardy to zone 4.

I wonder if you mean something other than rose of Sharon because I can't feature it as a hanging plant. Perhaps a tropical hibiscus? I will give you instructions for both just in case.

When the plant has finished blooming and the flowers fade, make sure all the flowers are removed. Protect the crown of the plant by either wrapping it in bubble wrap or covering it with mulch to protect it over the winter in your garage, Here's an article that will help: https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/rose-of-sharon-winter-care.htm

https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/wintering-hibiscus-indoors-winter-care-for-hibiscus.htm