The clone wars shows how the Jedi find new candidates. Once they discover a candidate, they go visit the family and talk with the parents. If the parents agree(and many of them do, as the Jedi are basically worshiped by the masses), the Jedi return at a later date, when the baby has reached a certain age, and bring them back to the temple.
The midichlorians is simply a way to test if a candidate has the potential to serve in the Jedi order. Everyone in the Star Wars galaxy has midichlorians. They "exist within all living things," but in rare case, there are individuals who have a higher concentration of them.
The midichlorians as a plot device was a way to explain how the ability to use the force can be hereditary(as in the OT), and how the Jedi maintain their numbers.
There was also a Darth Plageuis plot thread that was going to be part of Episode I, but was ultimately reserved for Episode 3.
Also, Qui-Gon tells Schmi that if Anakin had been born in the Republic, he would have been identified and brought to the temple. I think he was trying to comfort her in a way, as she was desperate for her son to have a life better than slavery, but that also tells us that Jedi candidates rarely come from worlds outside of the Republic.