What would be the best way to add more cats to our family?

You say your cats are young so it will not be a problem! The best thing to do is to adopt because it helps save millions of pets a year! It will not stress your current cats, do not worry! You should adopt cats from 1 month to 7 months because they get along better. They are always used to have cats around, they are used to it! Hope you adopt the cats, do not buy them!!