I need to make a jumbo rc truck with geared motor size (12inch /18 inch) & weight - 3 kg approximately
can some one please suggest me from where  i can buy the suspension for my truck?

Surajit Sir, I dont think Geared Motors would be a good idea for building a truck. Especially if you are using suspension. It ll be difficult to mount the suspension while using geared motors, coz generally geared motors are directly installed on to the wheels of car/truck. What we generally have is brushed motor, that are connected to Differential Gear including a gearbox that provides torque and also speed. Send me a rough design you are planning to implement, I will atleast try to search some parts for you.

Please share the specs and configuration of the truck. That way it will be easier to suggest the suspension for the truck. Also mention the scale.