go to a shop that carries fiber optic strands for bow sights. Take what you have and they can match up the diameter. Buy the color you like, take out the red and replace. They will explain about using a hot object to melt the ends so it does not fall out. I have done this on all my FO sights because I'm color blind and have issues with red. Easy fix! Good Luck!!
Not sure where you are at. I'm in Armstrong county and a local shop is heavy into archery. I'm not. I was in there and showed them and they had a big selection (Shultz's Sportsman Stop, Apollo PA). I would think any decent archery shop would have them. I do know that the sites for guns seem to be small than some of the FO used in bow sights. At least that was what I was told. I have changed mine a few times trying out different colors. I liked the brightest green. I also found some at a shop in Elderton PA along RT 422 (cant remember the name).