Can be bought from Legendarymotorsport.net for $1,000,000 in single-player or $150,000 in mutli-player.

Spawns frequently in front of Sessanta Nove at Portola Drive, Rockford Hills.

It will rarely be seen driven around Rockford Hills.
Can rarely be found parked in front of a house in Vinewood Hills

While you cannot steal Devin Weston's Adder from his house after Blitz Play (The 'enter vehicle' key does not work and the gate will not open anyway), it is possible to steal it at the beginning of I Fought The Law.... When driving up to the D symbol at the construction site, stop and get out on the footpath before the terrain changes to dirt (any closer and you will trigger the mission). You can see the Adder, Devin, Molly and another car clearly from there. Throw a grenade or fire an RPG at the other car, aiming to the left. If done right, the explosion will destroy that car, kill Devin and Molly, but leave the Adder (relatively) undamaged, and free for you to take. You'll most likely gain a two-star wanted rating in the process.