Right after you buy the car dealership you get a mission, and after that mission there’s an ending scene where you can get a car and bring it to a garage so you can sell them at your shop. Where is this garage?
Where [do I] find the garage where you can sell Wang cars in San Fierro
The Wang Cars showroom is on the same street as the garage. Keep driving a bit to the north and you should find it.
There are two ways to sell vehicles in GTA:SA
You unlock missions where you're tasked with stealing certain vehicles. These are marked in the map as if they were regular missions. There are four of them (Zeroing in, Test Drive, Custom Fast Track and Puncture Wounds). After completing these missions you unlock Wang Auto as an asset (that generates revenue passively - although you do have to collect the income in person). After finishing each mission another car is placed at the showroom, which you can take whenever you want
After finishing Cesar's missions in San Fierro garage you unlock the Import/Export missions. At Easten Basin docks there's a chalkboard noting down a list of vehicles that you can deliver in exchange for money. After delivering a vehicle you can import the same vehicle type in exchange for some money. There are three lists in total, unlocked after finishing the previous list.
It's worth noting that neither of them require you to actually store vehicles at your garage, apart from some missions where you end them at or pass through the garage. It is possible that you're talking about mod-shops, since in some missions you do have to alter the vehicle before being able to deliver it. If that is indeed the case, there is one next to Wang Cars (TransFender) and there is one on Western San Fierro (Wheel Arch Angels). There are no other mod shops in San Fierro, although there are more across the map (Consider consulting GTA Wiki for a full map)