I found a reference that gives the solubility of 1-naphthol in water as 866 mg/L, which is 6 mM. It's probably lower in buffer because of the ionic strength. Dissolve the compound in a water-miscible solvent such as DMSO at 50 or 100 times the desired highest final concentration for the standard curve, and dilute this concentrated stock solution into buffer at the desired final concentrations, taking care not to exceed the aqueous solubility.