There is a process where padawans are sent on a trial to attain their crystal for their first lightsaber. Saber crystals are only located in specific areas in the galaxy and they are not common, and the knowledge of where to find them is not common knowledge. There is an episode of clone wars that deals directly with this question.
As for Sith, it hasn't been revealed in cannon whether they find their crystals themselves, or they are made synthetically (think lab created diamonds).
As for being a jedi/sith to use a lightsaber, there is no requirement. However if you don't have the force, it would probably be disadvantage to use a lightsaber vs a blaster. So most people dont use a lightsaber. In the Episode III book, Darth Sidious didn't even like using the lightsaber, and thought the Sith should be able to use the force, and only the force to beat an opponent. However, due to his sick sense of humor he liked being the Jedi with their own weapon.