When fabricating custom parts, starting from raw materials gives you flexibility. However, it is time consuming and design changes can mean starting from scratch. It’s faster if you can find some part off the shelf to modify. For example, for custom optomechanics, we like to start with some part from Thorlabs or a similar company, and then modify it rather than start from billet aluminum.
This sort of middleground, between COTS and fabrication from scratch, is a convenient way to rapidly prototype complex systems. MISUMI offers a McMaster-Carr like online web site of parts than can be customized by them and then shipped to you within a few days.