Remember that bacteria tend to reproduce themselves! Therefore take advantage of that as much as possible for financial reasons! I occasionally purchase probiotic capsules and then use a capsule to produce yoghurt.
Kefir is meant to be stronger than yoghurt so well worth trying. You might also try other fermented foods. You could try sauerkraut, just the juice to start with as you might have problems with cabbage. It's best done anaerobically using a sealed jar or an airlock if you can find a way to do that without adding metals/plastics into what will be an acid environment.