Hi,
There is no reliable way to stock logic boards of iPhone's let alone ones from old models like the 4S. There is no factory on earth that produces them so you should think about your search as a search for a USED phone, that happens to miss all its other parts or has many broken parts. A completely shattered iPhone on Ebay or any other second hand site is a good bet. You could also try to search for the logic board on AliExpress but know that any logic board of an iPhone is essentially a second hand iPhone which got stripped of its parts. Finding someone who wants to get rid of their old 4S will always be a better deal than buying off a supplier that has to manually strip old (obsolete) phones of all their parts, and is trying to earn back their spent hours. Try to search locally, who knows what kind of treasures are hidden right next door. You'll be saving e-waste AND transport pollutions.