Even if you can't do RO, there are DIY methods you can utilize to manage this.

Since these drastic pH changes are caused by changes in the chemical content of the water, use ion-action water purifier filters. Get a large bucket, drill holes large enough to SQUEEZE some brita or pur or whatever filters in, seal them up tight with some sort of adhesive, set that bucket over another bucket (I drilled equal-size holes in a lid to allow the bucket to just sit on top while the water filters through to the second bucket below,) and pour your water in. The more filters, the more water filtered per minute.

Works with rainwater as well as long as you keep the bottom bucket sealed.