I'll speculate a little:
1) Good Screenwriters don't have to write on spec. They can be hired onto projects by the studios or financiers or they can pitch their ideas and get the money to write them. This is where indie producers who usually don't have development money really lose out.
2) Good Screenwriters command the attention of bigger, more successful producers who get the first pick of their work.
3) Good screenwriters are repped by agencies who ignore smaller producers, believing the project will not have a chance of getting made with a decent price tag or at all with a smaller producer on board.
4) Good Screenwriters can write on spec and get their scripts sold or perhaps set up for development at a big production company thus leaving the smaller producer out of the loop entirely.
5) Once successful, Good Screenwriters aren't in the orbit of smaller producers. Or they choose not to be.
Where does that leave the smaller producers who have no money or reputation to get a Good Screenwriter?
1) You find newer screenwriters to mold and shape into the Good Screenwriter. You make a film based on their freshman scripts and cross your fingers it's a hit. And when that Good Screenwriter grows up, you hope your relationship has them bringing their new projects your way.
2) You rattle the trees of agents and managers who will blow the dust off the scripts sitting on their shelves. Or take pity on you and throw a spec (usually one they don't love) your way.
3) You spend the time developing mediocre material into a gem.
4) You find and option books that the Good Screenwriters want to adapt.
5) You garner some success and go after the Good Screenwriters based on the heat from your success.
6) You befriend those who work at production companies with deals and you bring them material for Good Screenwriters. You then partner and they help you get that elusive Good Screenwriter on board.
7) You track winners of screenwriting contests and go after them to consider their material.
8) You hit up your friends in the industry for referrals to the up and coming screenwriters.
9) You attend festivals and screenings of material from new talent.
10) You come up with any and every idea to find sources of material. Your Aunt Gladys might even be a budding screenwriter.
You just never know where that Good Screenwriter is going to come from. That Good Screenwriter might even be you. The best thing is to keep your eyes and ears open and keep looking and read, read, read!