Why do you think you need to replace what you thought was a check valve?
What type of backflow preventer? It looks and sounds like it could be a reduced pressure zone type backflow preventer.
If it failed did the Inspector say you had to get it replaced? Or fixed?
If it's leaking, I'm guessing the leak is from the intermediate chamber, which is meant to leak/discharge water in the event there's a problem with it. Those things can be expensive but do have repair kits.
If that is in fact what it is, you have to get someone familiar enough with it to be able to test it and determine the problem. The plumbing inspector, if that's who he/she was, wouldn't necessarily know enough about it know.
So what's your real problem???
Just saw the above comment. Is that a water feed to a heating boiler, in which case it would be what is called a, "Dual Check Valve with Intermediate Atmospheric Vent", which is used on the potable water line feeding makeup water to a heating boiler. That is not a testable device.
Was this unit recently installed?
Why was it subject to inspection?