Sourcing is finding suppliers capable of supply. Issuing an Invitation to Tender (ITT) is a standard method. There will be on-line portals to publish an ITT. By tagging with keywords, suppliers receive alerts asking them to respond. An ITT is very specific and invites suppliers to submit their solution with prices. A pre-curser (comes before) to this is to use the same portal to ask for “expressions of interest”. This will generate more responses, no prices but allows you to choose which suppliers you take to “second interview”!
You can source by asking current suppliers, trade fairs, ask other contacts in the engineering network, asking google, chambers, ask any employees who used to work for a competitor.