How can I find the cars I buy on my character's cell phone? What garage do I go to to get the car?
The in-game help text told me that I can get them from the big garages that are available for each character. That would be the car park just south-east of Michaels house and the Groove Street Garage for Franklin. I've only found the hangar on the airstrip near Sandy Shores for Trevor, which seems only to be used for airplanes (and the nearby helipad for helicopters).
I haven't bought any car yet so I can only tell this from the help text.
Parking a car in a parking space can prevent it being towed/impounded + keeping track of the last location the car was parked will prevent loss of vehicle (and heart attack!) over loss of money and a sweet car.
In my case I assumed it would be at the impound lot when I finished a mission. I checked and it wasn't there - only 2 random cars I had no intentions of keeping were there.
Luckily, I was able to track down the starting location of said mission via Internet searches and was able to find my car.
It was not parked in the space I left it in as parking in a space sometimes allows the cars to still be at the location instead of the impound. I assume this has worked in the past because the car is not illegally parked, resulting in a tow/impound.