There are two garages where your mission is to deliver a certain amount of different cars. The first one is in the docks in Portland. The other one is on Shoreside Vale. There is a sign by the garage door with a list of what cars you must deliver. Some of these cars can be hard to find but you will encounter them in some missions. If you do that, it might be a good idea to fail that mission and drive the car to the Import/Export garage instead. The garages are marked on the locations maps with these icons:  

You will get a reward of $15.000 for each car you deliver and if you deliver all the cars on the list you will get an additional reward of $200.000 and pickup icons for all of the cars inside the garage so you can get any of those cars whenever you want.

PORTLAND  

Securicar
The easiest way of getting this security truck is to abort the Van Heist mission you get from Joey and drive the car here instead. You can also find it when you do you Firefighter missions.

Moonbeam
This van can be found pretty much anywhere in Liberty City. There is one parked at the parking lot at Head Radio in Portland.

Coach
This bus can be found in the streets of Portland

Flatbed
This big, flatbed truck can be found in the streets of Staunton island

Linerunner
A big 18-wheeler that is easy to find. There is one parked in the Portland docks right where you landed from the unique jump you do over the trailers.

Trashmaster
The easiest way of getting this truck is to abort the Blow Fish mission you get from Toni and drive the car here instead. You can also find it when you do you Firefighter missions.

Patriot
Looks like a Hummer and can be found almost anywhere in Liberty City.

Mr. Whoopee
The easiest way of getting this ice cream truck is to abort the I scream, you scream! mission you get from El Burro and drive the car here instead. You can also find it when you do you Firefighter missions.

Blista
A common family wagon-type car that can be found on the streets of Portland and Staunton Island.

Mule
A common truck that can be found on the streets of Portland and Staunton Island.

Yankee
A common truck that can be found on the streets of Portland and Staunton Island.

Bobcat
Looks like a small pick-up truck and can be found on the streets of Portland and Staunton Island.

Dodo
An red and white airplane with broken wings that can be found in the airport on Shoreside Vale.

Bus
Looks like a school bus and can be found on the streets of Staunton Island.

Rumpo
A common truck that can be found on the streets of Shoreside Vale and Staunton Island.

Pony
A common van that can be found pretty much anywhere in Liberty City.

SHORESIDE VALE 

Sentinel
A normal version of the Mafia Sentinel and can be found on the streets of Shoreside Vale.

Cheetah
A sports car that can be found in the streets of Shoreside Vale and Staunton Island.

Banshee
An open Viper-type car that can be found pretty much anywhere in Liberty City. There is one parked at the Easy Credit Autos in Portland, near 8-Ball's place.

Stinger
A normal version of the Yakuza stinger sports car. Can be found in the streets of Shoreside Vale and Staunton Island.

Infernus
Looks like the Cheetah and can be found in the streets of Shoreside Vale and Staunton Island.

Esperanto
A big Cadillac-type sedan that can be found almost anywhere in Liberty City.

Kuruma
A common sedan-type, family car. Can be found almost anywhere in Liberty City.

Stretch
The Limo. Can be found on the streets of Shoreside Vale and Staunton Island.

Perennial
A common station-wagon. Can be found almost anywhere in Liberty City.

Landstalker
A common SUV-type vehicle. Can be found on the streets of Shoreside Vale and Staunton Island.

Manana
A common car that can be found on the streets of Shoreside Vale and Staunton Island.

Idaho
A common car that can be found almost anywhere in Liberty City.

Stallion
A common car that can be found almost anywhere in Liberty City.

Taxi
The Taxi can be found almost anywhere in Liberty City.

Cabbie
The older style yellow taxi. Can be found almost anywhere in Liberty City.

BF Injection
This Beach Buggy type car can be found ouside the house where you picked up Misty in the Drive Misty For Me mission you got from Luigi. It only appears in the evenings.