My personal recommendation would be eBay, as for specific things like that, I've never had an issue. I had just gotten some SL SR ribbon cables off there for my brother's Joy Cons and they work perfect. Here's an eBay link for the correct one. https://www.ebay.com/itm/233046019743
This could be caused by a few things. First thing I would do is an iTunes backup and then a factory reset to see if it still has that problem. If it persists, that would imply that it is a hardware issue, and you can do a restore without worry. Rear cameras for the iPhone 6 Plus, even OEM ones, are cheap on eBay. If it fixes it, reinstall all your apps one by one and see if one of those caused it. Also, did you drop your phone or get it wet before this happened? If something specific caused it, it might be easier to diagnose what's causing it not to be able to focus.