There is no such thing as a free lunch, same applies to bonsai trees.
You can grow them from seeds, or cuttings and there is a cost, your time and effort.
You can also rescue tree or shrub stumps from skips and plant them in training pots and make them into bonsai trees. Again the cost is your time and effort. You can use old washing up bowls, make a few drainage holes and use homemade bonsai soil mix.
You can also collect material from nature, but must have the land owner’s permission to remove the material such as pines, hawthorn, blackthorn etc, the choice is endless.
Learn to make your own bonsai trees and you have free bonsai trees.
Watch and learn from my bonsai channel on YouTube: