the canopy is attached to the four posts of the bed and the four top rails by a wire or with four simple hooks one in each corner off the the four poster bed
Canopies are generally bought just for decorative appeal. In some countries canopies are used as protection against insects, especially disease caring mosquitoes, while they are asleep.
Bed Canopies are becoming very common these days. Thanks to the variety of designs like Disney Princess, Black, pink for girls. There is a major fan following for them. You can always look for them...
You can buy inexpensive bed canopies from sites like Ebay or Amazon. You can especially obtain a great deal on Ebay since you can bid on fairly low prices.