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There's plenty of maps available online that show you static spawn locations for almost every vehicle. However, if you want the fastest, you need to progress a bit into the Epsilon sidequest with Michael to get a few sports cars to spawn.
If you want a fast vehicle ASAP, just progress until after the first heist, then go find the Tinkle stock market random encounter with Franklin, get the 70% boost, go buy the Sandy Shores mod shop, then mod whatever you want for free. Granted, you'll have to progress pretty far in the story to unlock all the top tier mods, but you can still turn a fairly mediocre car into a racer with a few mods.