Online Component vendors like Digikey, Element14, Mouser, RadioShacketc. can be used for sourcing the components for the prototypes.
In the following article, we have compiled Ten Best Online Vendors. You may want to read that for effectively sourcing all your components.
The designer can bring in cost reduction in the prototype by analyzing the Bill of Materials (BOM) and selecting alternate components which have similar parameters but a lower cost.Also, the BOM should be analyzed for obsolescence. All the components in the BOM should have a guaranteed availability of 5 years and above. Some of the components would be readily available, but some would have a lead time for delivery. It is imperative that the component sourcing be planned in such a way that all components are available when the PCB arrives after fabrication.