According to Inouye et al, 2015 (http://dx.doi.org/10.1016/j.pep.2015.02.002), the coelenerazine-h is also a very good substrate for NanoLuc/NanoKAZ.
It can be purchased from many suppliers unlike furimazine.
Although, I didn't see a direct comparison. Furimazine, probably, has lower background.
PS. According to Promega (personal communication), Furimazine gives slightly higher initial bioluminescence and the signal then decays much slower compared to CTZ-h.
Thanks Michael, I tried using CTZ-h as the substrate for Nanoluc detection, it works. You can use regular renillar lysis and substrate buffer plus CTZ-h for nanoluc detection.