Why do golf balls sink in water?
A golf ball sinks because it is tired of being whacked with big sticks and has decided to end it all by drowning itself.
The golf ball sinks because it is heavier than the water it displaces. If the ball were put in a fluid with a higher density than ordinary water, like molten lead, it would actually float.
It is highly unlikely that you would be able to get golf balls for free. The only way you could do so is a promotion with a golf magazine or something. If you go play golf and find some golf balls you can keep them, however, you can't just go to a golf course and look for golf balls, this is illegal. The issue seems to be perhaps you think golf balls are too expensive…