Here are a few options:

1. Collect rainwater after its rained a bit (unless you fear pollution in your area or have a new roof). It's free and naturally soft and usually a bit acidic. Mix with a little tap water and you should have what you want;
2. Buy RO water and mix with a little tap water and you will have ideal water;
3. Buy high quality peat that the manufacturer lists as having a low pH value (check the bag -- usually only large bags say this - or call the company to check the acidity). The peat will need to be in a canister filter and be very careful as strong peat can lower pH and softness fast stressing/killing fish. The problem will be on water changes as the peat will have to take some time to resoften and acidify the new water. I'd suggest a water storage container to prepare the water for the water changes -- a second peat filter on that container too.

The rain water or RO can be mixed in a bucket during the change. Water softening pillows just replace the calcium in the water with sodium leaving the total dissolved solids in the water still quite high. It reads as soft water as the test kits check for calcium. If you test with a TDS meter or conductivity meter the reading would still show high mineral content: sodium and what not. Not really "soft" water.

In short, there's no easy way to change water chemistry. If you do, the fish will reward you with longer life, better color and more breeding but its a chore and a cost. Of course, you will need all sorts of test kits to play with the water effectively.