It's easy to make your own sourdough starter with any flour you want! The trick is to use kefir whey/water kefir/other ferment liquid as a shortcut. The kefir yeast and bacteria set you down the right path quickly.
Combine flour and kefir whey (or just straight kefir) to a pancake batter consistency. Roughly a cup is a good amount. Leave it on the counter and watch it over the next day or so. It should get bubbly, rise, and fall back down. You now have starter.
Feed it by doubling the amount of flour, and adding water back to pancake batter consistency. The more often you feed it, the sour-er and more active. The thicker the consistency/the more flour you add, the longer it will take the bugs to eat, and so the longer you can go between feedings. If you don't want to use it for a while, keep it in