You can buy sourdough starter (culture) on Amazon. We use to make starter years ago, my sister in-law kept a batch alive for around 5 years when she had a bakery. We didn't like sourdough all that much so we just stopped feeding it and through it out.
I know there's a lot of different sourdough start recipes out there, all I've ever done for 35 years is, 2 cups of flour, 2 cups of warm water, 1 heaping tbl spoon of SAF yeast & a tsp of sugar. I cover it & let it set for at least 24 hours, longer is better. My kids & grandkids ate SD pan cakes, SD bread, etc from this recipe for all those years & never knew the difference from a "good" recipe.
Made my starter from 1 cup of flour and 1 cup of water. Covered with cheese cloth and let sit on the counter for a week. That was in 1987. Still going strong. No metal ever touches the starter.