There is no cheat code that spawns a tow truck in "Grand Theft Auto: San Andreas." These trucks have to be found in the world, and they can be pretty rare.
There are several places to check for a tow truck. The first place is the Angel Pine Junkyard in San Fierro. The junkyard is located near Mt. Chilliad. Leave the town to the northwest toward Mt. Chilliad, and take a right. Just before the bridge on the right, there is a walled complex. That is the junkyard. Enter and proceed all the way to the rear left corner, where there is a tow truck. The tow truck can also randomly spawn in other places in San Fierro, including Doherty and Easter Basin Docks.