Safety of a Galaxy Note 7 with a non-factory battery
Given that the reason of battery failure is the incompatibility between the over-capacity battery and phone's battery containment space, manufacturers are unlikely going to produce batteries that are smaller in capacity than the original, considering they usually make bigger ones for profit and marketing purposes. They also manufacture them in generally worse materials and quality control in order to control costs. This renders those 3rd-party batteries (if any exists) as dangerous, if not more dangerous as the original one.

Also, since Note 7 has been recalled for quite some time, manufacturers have (nearly) ceased production of any sort of accessories for it - nobody's buying them anyway. Here's what I dug up from my local shopping site TaoBao (Chinese equivalent of eBay) - if you can't read Chinese, the page shows that only "original" batteries are available, and even those are available in very small quantities (few resellers list them).