You can order them online. If you search for hibiscus tea you'll be directed to sites that sell the dried flowers by themselves along with herbal tea blends containing them. You need less than 1/8 oz. for a cup of the tea, so if you were brewing this as a full strength tea, I'd say half a pound for 5 gallons would be way more than enough.
The idea came from Jolly Pumpkin - they do a red saison called Baudelaire brewed with rose hips and hibiscus. They're usually pretty secretive about their recipes, but you could try to find out from them how much they use.