If you are using 100% rainwater, you don't need to use water conditioner
There are 2 reasons to use it, to remove chlorine and to bind metals, and there shouldn't be any of either in rainwater.
So for cycling you can use bicarbonate of soda to boost the KH. You may already have some in the kitchen; if not it's in the home baking section of the supermarket. Scoop out a bit of tank water and add bicarb to that at the rate of 1 level 5 ml spoon (or teaspoon) per 25 litres tank water, and when it's all dissolved pour the water back into the tank. Cycling makes a lot of acidic things (nitrite & nitrate) and these will cause the pH to fall as the cycle progresses. Bicarb will stop this, but it could get used up so it's safer to test pH whenever you test for ammonia & nitrite. If the pH starts to drop, add more bicarb at the same rate.
At the end of the cycle, a large water change is done to remove all the nitrate made during the cycle and this will also remove the bicarb before you put fish in there.
Then it's just patience.