I've been getting my ferro rods since forever from Going Gear. Always great quality.
Survival - Going Gear
I just checked the link you posted for Nathan on ebay. He has really good prices if the quality is as good as you say. Definitely worth a check.
I standardized on 3/8 x typically 5 or 6 inches so all my sheaths are made to the same spec. I find the 3/8 is more than enough and the length is the key. On some I add a handle or drill a hole to add paracord. This cord actually has glow-in-dark fibers that I made for one of my son's set-ups.