I too had all of your concerns when I started. So I experimented with spare grains and using food grade plastic, like the kind sold for brewing. The results were fine and I use regular milk from the refrigerated section of my local market.
Glass is great and certainly better than regular leachy plastic but it can be heavy and discourage some people from trying to culture their own kefir. One day I hope to make kefir from raw cow and goat milk as I hear it is especially good.