Mr.Woopee

1: Do Firefighter or Vigilante Missions and it'll appear evetually. (I found one after my 35th Firefighter call).

2: In one of the jobs you do for El Burro, you'll need to use a Mr. Whoopee truck to lure out some dudes in the Atlantic Quays that are threatening him. Normally you'd plant a bomb in the truck and blow it up (from a safe distance) when they surround it, but instead of doing that you can just whip out a gun and shoot them up to complete the job, then get back into the truck and take a short drive up to the docks.

3: This is just a rumor, but supposedly if you hang around in front of the Cartel Mansion in Shoreside Vale during afternoon hours, one will show up. Of course this is unfounded, and risky because of the Colombians that are crawling all over the area. But if you wanna try it, then knock yourself out...
 
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