How do commodity traders secure new sources of supply?

So if we’re staying with the large scale energy landscape, from my experience a supplier looking for offtake, or a buyer looking for supply, will issue an RFP/RFI to market participants they have a relationship with, or more publicly, which offtakers/suppliers then respond to if they’re interested. For more regulated landscapes like natural gas or power, often offtakers will need to pre-register with the relevant regulatory entity (pipeline owner/utility), do things show they can post credit/collateral, are authorized to participate in certain markets, and can manage the logistics. Then the supplier will issue the RFP/solicitation to these pre-registered parties.
I think if you’re asking how do things start even before that, its really case by case. Many of these relationships have been around a long time. The oil majors are old, utilities are old, etc. The better thought experiment would be a new entrant, maybe an international player or acquisition, in which case your buying those pre-existing relationships or regulatory approvals. Otherwise ya it does just boil down to calling people. Here is a somewhat tangible example, since I’m bit hard pressed to think of one in oil/gas in this vein: a former employer is an early entrant in the hydrogen/e-fuels business. They needed to get water/carbon (I think, it could have been another gas) to make an e-fuel like methanol to deliver to an end user. The water they could get by buying the right land, but carbon is harder. So they were digging around an area of the country looking for industrial facilities with excess carbon they could purchase that wasn’t fossil fuel related, such as paper mills. And I’m pretty sure just hitting the people up on LinkedIn to set up the initial meeting.

The other thing to be explicit about just in case is re: the original question about how traders get supply, in energy at least its more about how end-users get supply. And often times a “pure”, middle-man trader who owns no physical assets isn’t really in that physical delivery process. Traders definitely play a role in hedging, liquidity, and speculation around the physical process, but mostly the supply owners, demand owners and transmission owners have some system worked out between themselves, because the infrastructure is so expensive and long lived. So some “traders” take on this hybrid, physical asset owner role in order to participate.
LNG is the main exception I can think of right now. The commod houses do have a big role in moving that, somewhat similar to their involvement in oil.
But definitely someone in ags/metal should chime in because anecdotally I’d think the picture can be different because storage, transportation, and physical risk is really different (ie oil spill vs sugar spill).