Sprouting seeds doesn't take much water. For right now you could use bottled, distilled, etc.
Calibration solution is cheap. An 8 oz. bottle is about $10, and 8 oz. would probably keep you until the cal solution expires. From my experience cal solutions have about a 2 year shelf life. Single stage pH 7.0 calibration would be good enuf. You could do 3-stage cal but probably not necessary.
Or, go with MickFoster's suggestion and buy a simple pond/aquarium pH test kit.
I agree with Mick's comments about humidity domes.
Earlier I mentioned your tap water pH. I forgot to mention that tap water pH often takes annual swings. Not by huge amounts, but it'd be good to know what you're getting year-round.