You can buy the spring itself from many places. Fishusa.com or Reeds should have them as well as Gander of Cabelas.
A you need is an 1/8″ ID Rubber Grommet from the local hardware store and a #8 one legged rod guide.
Make your own using the above. Super glue the rod guide to your rod tip to hold it in place, now glue it with two part expoxy. Insert 1/8″ rubber grommet and spring.
You might want to use an 1/8″ drill bit by hand not in a drill and slightly enlarge the opening in the grommet. The spring needs to move in the grommet to adjust the sensitivity but it also need friction to hold it in place.
If you have any questions let me know, I have made dozens of these for personal use of course.