This is a Filament Wound Epoxy Tube (FWET) with an outside diameter (OD) of 0.524" that works well for Blue weapons and short Green weapons. It's available through Goodwinds - shipping is expensive because of the size, so you may want to order with a friend.
1/2" Type K Copper Tubing has an inside diameter (ID) of 0.527" and works well to slide over this tubing, for making modular pommels and attachments. Other, steel tubing has the same ID and works well too, but isn't as weather-resistant as copper.