Have you checked your local Yamaha dealer?
If not available there go to the hardware store with your old one in hand, find a spring of about the right wire size and diameter, and don't worry about length you can easily make it shorter. Cut the new spring so that it has a bit of tension with the arm fully retracted. Install cover (watch out for that pesky ball bearing in the worm). Go for a ride.
Mission accomplished, go have a beer.