There are a few ways I know of to raise the pH. The first is to aerate it. Part of the acidic nature of rainwater is CO2 concentration. Collect your rain water, place it in a bucket overnight with an aquarium aerator running and test it again in the morning.
Another way is adding baking soda, might not be the best way because it will likely cause white water spots on the glass fairly quickly.
Anything else takes a long time that I have used for aquariums and would add minerals anyway.

You could always distill your own water. I have a cousin that does it for making beer. I don't know how much the initial investment would be. I would have to research it myself.