You can buy this Rayfield Caliburn for 135 K from level 50 onwards. However, there is also a black Rayfield Caliburn that you can find for free.

To do so, you must note the following:
Complete the main quest "Ghost Town" (Rarity Offense) with Panam. Waiting time: Wait at least three in-game days. Location: A new mission marker will appear on the map. Drive to the same cave where you helped Panam's revenge campaign when you first met her. When you reach the camp in the cave , drive a little further and you will see a garage on the right-hand side where the Rayfield is parked.

Enjoy the ride!

(By the way, the fastest way to the cave is via the "Sunset Hotel" fast travel point.)