[cogman10]

Easiest place to get it is from the product you want to produce.

For example, if you want to make yogurt then grab a little bit of the leftover yogurt in your fridge, drop a dollop of it in, and viola, it'll start the yogurtification process.

You can also rely on the open-air bacteria for some culturing, but the results can be all over the place. This is how a lot of sites suggest starting sour dough.