It might be harder to get a good starter for these in the US. Keeping a yoghurt culture in good health requires commitment too. It’s like having a pet, it needs regular maintenance.
a friend of mine did that. unfortunately he switched to making beer which doesn't mix with kefir before i could get his culture.
i found yoghurt rather maintenance free in comparison, especially because it is just the yoghurt, which can be kept fresh in the fridge for a few weeks and frozen for longer. it's also easy to replace by fresh yoghurt from the store. so rather worry free.
kefir culture on the other hand is a separate beast and not the resulting kefir that you consume.
the kefir culture needs to be taken out of the kefir when it's ready and put into fresh milk or it dies.
the culture also grows so you get more of it to give to your friends.
it's rather like tending to an alien animal of sorts.
watch it grow after each iteration. gently rinse it before putting it into fresh milk...