No matter where you live, rainwater has always been slightly acidic due to the CO2 in the atmosphere. Here, in Scotland, the water supplies from the hill can sometimes be considered too acidic (it corrodes pipes and fittings) so the water treatment companies use lime to reduce the pH. I don't know of anything suitable for garden use. You could place a block of limestone in each butt, though I doubt it would do much.

As Bulbaholic said, lime is probably your best option to increase the pH level. As he quite rightly points out, rain water can be slightly acidic. You also said that the water is borderline for acidity. So I don't thin that you would need much addition of lime to swing it the other way, i,e, slightly on the alkaline side.