You do not necessarily need an additional backflow preventor as your main line to your house should contain one at the connection source from the city. On my installs I always install a check valve for the portion of the line supplying the sprinkler system, just as a precautionary to keep "dirty" water from backing into the main line to the house. The check valve only opens up to allow water to the sprinkler system when the system is on.
Andyb, Ace Hardware sells an exact replacement for this valve box... You can order it online from: http://www.acehardwareoutlet.com/(e33priyzneu3jtembml0r155)/ProductDetails.aspx?SKU=4026597 EDIT: The link does work.
