Firesteel.com. Lightmyfire are good too. Things to note, the easier and hotter the sparks, the more reactive the rod, so the more it will corrode with time. In my wet environment that's not ideal, but I make due.
There are not very many places that actually do the manufacturing, so while a place like firesteel.com or LMF have a desired formulation and consistent quality, the other ones on the market can somewhat be treated as seconds. If you are practicing, buy a bunch, get them cheap, odds are they are all coming from one of only a couple factories. If you are packing kits, I'd go with a reliable vendor and have confidence in what you bought. I've found that random ebay steels are better when they are just blanks, but that's just been my luck.
The other key is the striker, if nothing else if you can get the Firesteel.com carbide strikers, they are well worth it.