Hello. First, Codefactory has a SAPI 5 version of Eloquence that you can get for a price. I don’t have the price on the top of my head, but if you google Eloquence for NVDA code factory Eloquence. Another add-on they have is the Eloquence and Vocaliser expressive add-on for NVDA which also Codefactory makes, and it contains Eloquence and Vocaliser in one add-on. But as for free voices, the only free voices that I know of are TruVoice, and the Microsoft Speech from 1999 with Mary, Mike, and Sam. A slight correction. I think you meant windows one core voices. If your' on windows 10, NVDA doesn’t use ESpeak as the default voice. Instead, you get the one core voices by default if your on windows 10.
I hope this helps. Thanks, Sky.