Star Citizen ships may be purchased either with in-game currency (aUEC) or with real cash. Ships purchased with cash are called "Pledges" and are permanently attached to your Star Citizen account. These purchases help fund the development of Star Citizen and Squadron 42.
Pledge ships can be bought as:
game packages include access to Star Citizen with the ship.
standalone purchases result in a ship being added to an existing RSI account, but does not give access to Star Citizen (most useful for existing backers).
Ship Upgrades replace a ship that is currently on an account with a more expensive ship while charging only the difference in value between the two ships.
To purchase ships using real currency, see: Pledge Store
Availability
Ships have different availability for various reasons, set by CIG:
Always available - Common
Time-limited - Uncommon ships - put on sale from time to time.
Quantity-limited - Rare ships - Batches of ships offered for short periods.
Promotion only - Only purchasable through promotion offers
Not available - Certain alien or military exclusive ships.
With all ships being earnable in-game on release, there is no need to spend real cash on them, unless you wish to support the game, and even less so any need to purchase limited ships on the grey market.
In game
Ships can also be purchased in game, using aUEC earned from in-game activities. These purchased ships are fully available for modification, painting, and destruction.
Note: Occasionally, the developers may "wipe" all aUEC-purchased ships as part of the Pre-Alpha testing phase. These wipes are generally announced in advance.