That is a good, informative and generally accurate page that Jim March linked to however this bit right here is just plain wrong:
"If it was a city or county cop, make a THEFT complaint in detail with your nearest California Highway Patrol station (they investigate local wrongdoing). If it was CHP, hmmm...complain to the CHP supervisors maybe, or the Sheriff, but for God's sake don't let 'em off clean."
The CHP does not investigate "local wrongdoing" at all, they investigate traffic collisions, traffic offenses, DUIs, stolen vehicles, etc. If you have a complaint about a particular officer seizing your property for evidence, contact his supervisor. If you are still not satisfied, contact that person's supervisor and so on until you hit the Chief or Sheriff(or whatever underling of the highest rank that will see you.) Still not satisfied? Start in on the City/County officials, like councilmen or supervisors and such. Still no satisfaction, contact an attorney and start legal proceedings.
And please, forget about the FBI or other federal agency, they aren't going to investigate some street cop who took your pocket knife.