Canning jars are the least inexpensive container to use in making water kefir. For most purposes, a quart-size canning jar will be perfect for the first stage of making water kefir: inoculating the sugar water with beneficial bacteria. If you are making small batches, a quart jar may be adequate in the second stage as well. A half-gallon jar may be a better bet as your water kefir projects grow.
You can find quart-sized canning jars fairly readily at second-hand stores and yard sales. Half-gallon jars are more difficult to find used but can be found at canning supply stores, including the nation-wide craft chain Michaels.