Bacteriostatic water has become popular recently. Saline was the original choice. But bacteriostatic water is much better at stopping bacterial growth, so for longer storage after mixing, it's preferable. Only certain high-quality versions meet standards, so those are ideal when available. Otherwise, tests indicate that some online sources don't work well, and others can cause pH problems leading to gelling in some cases. Thus, pharmacy-grade saline is a reasonable alternative if the preferred option isn't accessible, but limit use to 20-30 days.
I'm wondering if people use saline rather than water for this. Previously I'd mix a bit of benzyl alcohol into saline for bacteriostatic properties, though it was tedious to filter and measure. The main purpose is nasal sprays since the body handles saline better than pure water.