It is very simple to make your own sourdough starter. All you need is time and patience.
bread, sourdough bread, sourdough starter
active sourdough starter
wholegrain rye flour
Start by putting 100 grams of rye flour and 150 grams of water in a glass container
Mix thoroughly so all the flour been hydrated
Put somewhere warm to the next day (in a window in the sun or near a heater)
The follow the routine beneath until you have an active sourdough starter. You know it is active when it grows to at least double size over four to eight hours after you've fed it
Mix the sourdough starter
Measure out 50 grams of sourdough starter in a fresh, clean glass container
The starter that's left over should be thrown out, at this point it may contain bacteria that you don't really want to eat.
Add 100 grams of bread flour and 100 grams of water and mix thoroughly. Close the container. You can seal it if you want. Store somewhere warm (about 25°C/77°F)
Repeat every day until your sourdough starter is bubbly and grows predictably. Normally it takes six to seven days
Every day when you are about to feed the starter, you might see a brownish liquid on the top of it. Just stir it back in.
The starter should smell sour, it doesn't mean it has gone bad.
Any mold or mildew on top of the starter means it has gone bad. There is no way to save it.
https://foodgeek.dk/en/make-your-own-sourdough-starter-recipe/ - Sourdough starter