Clarification: even if your valves have backflow preventers, if you have a hilly landscape and the backflow preventers are not higher than the sprayers/drippers, you still might want to have a central backflow preventer (because gravity back pressure can keep the BFP from venting). A dedicated BFP has forward and backward pressure release. Mostly an issue if you are using manure or chemicals on the irrigated landscape - since you don’t want that re-entering your house or city drinking water.
Also, I find the integrated BFPs always leak within a few months, which is not great. I prefer a central BFP and simple valves with no BFP.
Amazon has BFPs like this: https://smile.amazon.com/gp/product/B0012QGSKC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1.
BEWARE that many vendors including Amazon still sell parts like this that have lead in the brass fittings. California (which it sounds like you are in) REQUIRES lead-free devices. The link above is for a lead-free BFP. If in doubt, make sure it is listed for DRINKING WATER.