To get a Ralts, you have to finish the Yancy/Curtis side quest. The third trade is a Ralts. That's the only way to get that line in B2W2, and the quest is able to be done before the E4. It is a very difficult and easy to mess up quest.
Bronzor and Bronzong can be found at the Abundant Shrine. In order to access the Abundant Shrine, you must obtain Waterfall. N gives you Waterfall just prior to entering Victory Road, so you are barely able to access Bronzong before the E4.