Astral Boards are a rare and expensive (at first) method of transportation. In order to use an Astral Board, you have to finish the quest granting you the ability to ride them. The next step is much more difficult, as you need to FIND an Astral Board. Most players who would sell them, sell them for an incredible amount of money, but it's the most reliable way to get one. Some enemies in Fort Ruina have a small % drop for Astral boards, as long as you don't mind farming and fighting for enemies among groups of bots. (One way of solving this is to kill botters, but they'll soon reappear.)