If you don’t already have a sourdough starter, and want one, I always have packets of dried starter on hand. To get one for just $5, just place an order here. I am now also selling LIVE starter for $20, if you want to have one ready for use in just a few days. (I’ve done this a few times successfully, though because I cannot 100% guarantee it will survive the trip, I also include a packet of dried starter as a backup.)
The starters will come with instructions for reviving them, but in case you need them:
Reviving your dried starter is simple: Just combine the contents of the packet with 30 grams (2 tablespoons) room temperature water and 30 grams (3 tablespoons) bread flour. Cover and let sit at room temperature until bubbly and expanded by at least half, 12 to 48 hours (check every 12 hours or more). After that, continue to transfer 15 grams (~1 tablespoon) of your revived starter to 30 grams of flour and 30 grams water once or twice daily, until it about triples in volume in less than 8 hours. After that, you can bake with it and store it in the fridge as described here.
Reviving your live starter is simple: Add the entire contents of the container to 60g bread flour (6 tablespoons) and 60g water (1/4 cup), cover and let sit at room temperature until about tripled in volume, 8 to 24 hours. After that, continue to transfer 30 grams (~2 tablespoons) of your revived starter to 60 grams of flour and 60 grams water once or twice daily, until it about triples in volume in less than 8 hours. After that, you can bake with it and store it in the fridge as described here.