Not sure about the Seattle area, but every supply house (Ferguson, Winnelson, Kemper, etc.) here in Cincinnati carries a dual check backflow preventer. Ask for a 3/4" Watts Series 7 dual check valve.
As far as the silcocks go, you can install a hose connection vacuum breaker on them.
Check on Watts Regulator's website. www.wattsreg.com/products/
You can do a search for the Series 7 and the vacuum breaker. I tried to post links to both pages, but for some reason I cant. Both products are there however.