I use buttermilk quite a bit. I leave it out at room temp for 8 hours or overnight, then pour it into ice cube trays and freeze it. It will be thick like yorgurt. When I make a 10-11 litre batch of cheese, I usually use 2 or 3 ice cubes of it (add them usually when I start to warm up the milk and they melt as it goes). However, go with the culture you have. If you've got some buttermilk, but not thickened, just add 50 mls or so along with the culture you have to mix a few extras in there, then ripen the rest and make starter cubes from it. (Just make sure you sterilize your ice cube trays! Oh yah, and store they cubes in freezer bags after they are solid, not open in in the trays).