Here's a link to where I get my "raw" fiber rods. They sell them by the foot and as mentioned I feel that 3mm is the minimum for front sights:
http://shop.fiberopticproducts.com/...ent-Fiber/Fluorescent-Fiber-Gun-Archery-Sight
http://shop.fiberopticproducts.com/...ent-Fiber/Fluorescent-Fiber-Gun-Archery-Sight