Other

A few things to get out of the way:

No, I will not recommend PVC. You won't like it if you use it. Wood generally sucks for weight and is less safe than other options, but people have had good long term experiences with Bamboo and with Hockey Sticks (though the latter would make for a very hard hitting weapon and I recommend against it). Broom handles, dowels, etc, should be avoided.

With the prices we can get Carbon Fiber for, now, Bandshoppe is also not a good recommendation.

Graphite

Generally, graphite cores are graphite golf clubs or similar that are being upcycled into being a core for a foam fighting weapon. These cores are often relatively short and are tapered. The weapon's made from them are lighter than any other core option can produce, but they are also more fragile and generally don't last as long as kitespar or carbon cored weapons. Don't bother ordering graphite; if you're going to place an order, carbon is also light and is much more durable.

Sources

Graphite Golf Clubs	Thrift Stores, DAV, Salvation Army, et al.

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Bamboo