Kia ora! Agree with all the above advice I would definitely try and direct market into councils as Jonathan suggests, the stones could be used on walks and trails, council buildings and landmarks etc
Also as Vicky notes telling your story with targeted media which could include environmental titles as well as those targeted at an older market, would help generate interest. Please let me know if you would like some tips on how to approach.
I would also consider direct marketing into Māori and Pasifika communities, as headstones or kohatu are very important culturally. Tipene Funerals could be a good business to approach in this area, but you could also try to make direct approaches into iwi and hapū. However this would need to be done carefully. What area are you in? Perhaps you could start to build a relationship with your local iwi? Your story I’m sure would resonate. Good luck and please get in touch if you need more help / tips