Finding an old iPhone 4S, even if it’s broken, is often the best way to get a working logic board. Have you had any luck searching locally or on sites like eBay, Craigslist, or Facebook Marketplace? Sometimes repair shops also have old parts lying around.
If you’re looking for a functional board, make sure it’s iCloud unlocked and not blacklisted. Let me know if you need help figuring out the best way to test or verify a board before buying!