Promega usually recommends 1:100 dilution of "Live cell reagent" (catalog # N2011) which gives you about 5.8 μM furimazine. Promega also has a reagent for in vitro assays (catalog # N1110) which has a stock of 3.42 mM furimazine.

You can also use bis-CTZ which actually even has a higher activity with Nanoluc than furimazine. It is also much cheaper and can be purchased as a pure compound rather than a not strictly defined reagent solution. Bothe bis-CTZ and frZ are structurally very similar compounds (furyl vs phenyl side group).

1️⃣ More details can be found in the slides here: https://www.linkedin.com/posts/mkoksharov_the-first-proper-luciferase-based-calcium-activity-7314657803254521856-J_pN

2️⃣ Method details (on how to properly dissolve CTZ-derived substrates and how determine their concentrations) can be found here: https://www.protocols.io/view/chemiluminescence-of-coelenterazine-catalyzed-by-c-5qpvoy7n9g4o/v3/materials