I suggested a motor drive (speed controller) but the OP said that it's too expensive and he only wants a soft starter. While I don't know of a soft starter for 12VDC motor I have used them for 480VAC 3 phase motor often. The application of soft starter is for the case when you would want to run the motor at full speed all the time and just want to limit the inrush current. Golf cart and most battery operated vehicles have motor drive to control their speed. You wouldn't want a golf cart to run only at full speed do you?
So when someone said to use a controller from the golf cart I asked do golf carts have soft start and not speed controller?