I need a slow flow sippy cup for my son?

my son is 12 months old and my nephew is 7 months old and i can't find a sippy cup that flows slow enough for them my cousin and i have tried nuby, avent, and gerber grads. and those are way to fast for our boys i don't know what else to try if they make like a different valve for sippy cups like they do bottles or what can somebody please please help us? thank you