You can make saline solution easily at home. You boil half a liter water with 1 1/2 table spoons salt.
Let it boil until it dissolves. Any type of salt works here,
but sea salt is the absolute best for this purpose. Then you
pour the saline solution into a clean container that should
not get contamined with bacteria. Consider making smaller
batches of saline solution instead of one really big.
This way the saline solution keeps fresh and clean.