It's pretty simple. Here's what I do. For buttermilk, I just buy a 500 ml carton of live culture buttermilk. Then, leave it out at room temperature for the day. It will thicken like yogurt. Pour it into sterilized ice-cube trays, freeze, then remove and bag the ice cubes.
For other cultures, like Flora Danica, I buy a 1 litre UHT skim milk. Put a small amount of culture in, and again, leave it out for 12-24 hours. For a thermophillic culture, I put the carton of UHT skim in the hot water cupboard.
I've used ice cubes that are a year old, and they seem to work fine. Typically, I think 3 or 4 cubes works well in a 10 or 11 litre make.
Do a search on mother culture and you may find more.