Any car that's shown as "Not For Sale" means that you must win it, or achieve it by passing missions. There's an FAQ at the top of this section of the forum listing all the possible prize cars for your version of the game. You can also buy it from someone who has it on their own game save, and thus, trading it from another memory card.
Incompete specs are usually because the information about the car wasn't available to Polyphony Digital, or it's a "fantasy" car that doesn't really exist. But in any case, you can find out the power and weight of any vehicle by using the drop-down boxes and selecting "power" or "weight", even if the specs are hidden.
Many prize cars have no value; they can be bought from another memory card for 200,000 credits.
Edit: G.T beat me to it.