Buy from a Traveller
On your journey across the wasteland, you’ll find travelers offering you fine wares and goods to make survival that little bit easier. However, there are reports that some travelers out in the wasteland have Brahmin for sale that can be sent to a settlement of your choice.
Just like the traveler who was willing to sell his dog, all you have to do is stumble upon this traveler by chance and you’ll hopefully be given the opportunity to buy the strange mutant cow. Just make sure you have the money to afford the creature. Once you’ve paid him, simply select the settlement that you want to send the Brahmin to and it’ll slowly but surely make its way there. Sometimes fast traveling and waiting can help to make the Brahmin appear at your chosen settlement.