I'm very new to fermenting and am typically doing it with vegetables pressed under either salt or salt and water.
I've come across several sites teaching about how bacteria cultures can be used in this process, but am unsure how to create them and store them instead of buying them.
Unless you've got the time and resources to set up your own biology lab, you're not likely to have much luck raising your own bacterial cultures from scratch. You'd probably need growth mediums suited to particular strains of bacteria and strict isolation between them to prevent other opportunistic bugs from taking over. If you really, really want to try, you might have better luck asking on another SE site.
Fortunately, a lot of pickled-vegetable recipes (such as this recipe for dill pickles, or this kimchi recipe) don't bother with an initial culture. This is what's called a "wild fermentation", meaning that you're culturing with whatever bacteria happen to float their way into your fermentation vessel. If the initial culture is present, it's there to kick-start the process, or to obtain a very specific flavor profile by using a precise blend of bacterial strains. In those cases, you'll want to buy, because it's practically impossible to invite only a single strain of bacteria to the party. Even if your kitchen is scrupulously clean, exposing your food to open air also quickly exposes it to bacterial colonists that you can't control.
Other fermented foods can be started from a purchased culture, or they can be inoculated from a sample of active culture in a purchased food. For example, you can use store-bought yogurt with live cultures as a starter for fermenting your own, and you can cultivate the scoby from a store-bought bottle of kombucha to make your own at home. With some care and maintenance, you can make some of these cultures last for years. The cultures that give sourdough bread its distinctive flavor are sometimes kept this way.
If you need a specific starter for your recipe, you'll just have to pony up and purchase it. Depending on the final product, you might be able to use a different starter and wind up with a similar end product (see here) but it would be inadvisable and possibly unsafe to deviate from your directions.
There is typically no need to do that. Sometimes you can use an existing cultures to accelerate fermentation (use some liquid from the sauerkraut batch that just finished fermenting in the new batch for instance).
Other times you can keep a colony of bacteria alive for a long time. I've kept my sourdough culture going for 3 years before it died in the fridge after I took a long break from baking.
If you create the right conditions, you'll naturally get a culture of bacteria you want in whatever you want to ferment. Most of the time, lactic acid bacteria and/or yeast are the microbes used for fermentations. These are literally everywhere, so it's easy to get them into the food.
