About the core rod: I found an online supplier and ordered both 4mm fiberglass rods and a 3mm square carbon fiber rod to try out. The carbon fiber is stiffer and much lighter than the fiberglass. It should be stronger, too, which is why I got 3mm instead of 4mm. I used the one (it's pricey) carbon fiber rod on this sword. The fiberglass, on the other hand, is much twangier, and also heavier. A lot of people find my swords a bit too lightweight, so if you're someone who likes your weapons a little hefty, by all means, try out fiberglass. Do be aware that heavy means you'll need more padding.
If you want to know more about how I made this, I made an I'ble that explains the methods behind it.