I have a protocol that we used some years ago. The original comes from an article by M.Mesulam in J.Histochem&Cytochem,26,(2),pp106-117,1978.
1.Rinse in dem. water.
2.Pre-incubate for 20 min at RT in a mixture of 100 mg sodiumnitroferricyanide, 5 ml 0.2 M acetatebuffer pH 3.3, 92.5 ml dem. water and 5 mg TMB in 2.5 ml abs. ethanol.
3. Add 2.0-4.0 ml 0.3 % hydrogenperoxide and incubate for 20 min at RT.
4. Stabilize reaction for 20 min at 4 °C in a mixture of 9 g sodiumnitroferricyanide in 45 ml dem.water, 50 ml abs. ethanol and 5 ml 0.2 M acetatebuffer pH 3.3.
I must say we didn't use it for ELISA but maybe it could give you some idea's about the concentration, solubility and pH of the TMB solution.