Cal. Sts. & Hy. Code § 5584
If a county is conducting the proceedings under this division the legislative body may also purchase all or any part of the materials to be used in doing any of the work mentioned in this division and furnish the same therefor. The purchase may be made from such funds of the county as the legislative body may designate.
If the county has a purchasing agent, the materials shall be purchased by him; otherwise the materials shall be purchased by the legislative body by contract let to the lowest responsible bidder after notice calling for bids has been published at least five days in a newspaper of general circulation in the county. If the county is to furnish materials the resolution of intention shall so provide. When such provision is made, the execution of the contract for the improvements shall create a liability on the part of the county to furnish the materials provided for in the resolution.
Ca. Sts. and High. Code § 5584