Is the fothergilla shrub deer resistant?
Unfortunately, no it is not.
This article may help you: https://www.gardeningknowhow.com/plant-problems/pests/deer/deer-resistant-plants.htm
When should you trim Fothergilla
You really shouldn't. At all.
If you must prune, it will be best done after flowering, or before buds develop for next years flowering.
Here is an article that will help you to care for the shrub:
Where can i get it?
Unfortunately, we do not recommend places to purchase. I would say that you could try your local nurseries, or online. We can offer information on the plant, though: