Jacob, I am afraid I cannot say anything about American brand names of mixed-bed-filters, but maybe Bill Little is able to do so.
As your other question is concerned: Yes, rainwater is good, some say: it is best. Today, the rain is often falling down with an acid character already because of the intensified CO2 and other acid substances (from industrial emissions) in the air. We did not encounter any problems by that. What you have to obey is that the rain should not be used in the first hour especially in industrial regions; there should have been a certain “washing” of the often polluted air before you use it. And it should not run across rather dirty roofs or other materials before being collected.
But if you obey such simple rules, rain water is very good indeed. And cheap!