Plenty of sellers on Etsy offer sourdough starter, too.
If patience (or yeast) is in short supply, you can also buy starter from reputable online sources like King Arthur Bread, a website I recommend in general for its gentle tone and flour expertise. If you have a local bakery, you could also ask them to sell you a bit. Plenty of sellers on Etsy offer sourdough starter, too. There's even advice for making a gluten-free starter, if you love bread but your body hates gluten.