I've made a lot of buttermilk, as I use it for drinking and for cooking. If there's a store-bought buttermilk that you like, use that as your starter and then re-culture from batch to batch. I've got two local brands, but I don't know what cultures they use except that one makes a thinner and more acidic buttermilk and the other makes a thicker and less acidic buttermilk. They both re-culture well. I culture my butter with MM100 or Flora Danica, and the buttermilk that comes from that is very very good, but seems to separate easily and is grainy. I've tried re-culturing it and those tendencies become more pronounced. I've also used sour cream as a starter for buttermilk, which seems to have L. lactis and L. cremoris, and that works well and re-cultures well. I try to keep it at around 76f or a bit warmer to culture - warmer than that it seems to get an uneven texture. By the way, when I use MM100 or FD for sour cream, the texture is excellent and doesn't separate or go grainy.
Dave in CT