The (H^+) for a weak acid/weak base salt is (H^+) = sqrt (KwKa/Kb). So you look up a Ka that is smaller than Kb. You know Kb is about 1.8E-5 so any acid that is smaller than that should produce a basic solution. Look in your Ka tables for an acid smaller than 1.8E-5 and if the NH4^+ makes a stable salt with that you have your answer.