I'm actually currently making her spear and I bought a PVC pipe from the hardware store to be the staff part. It's pretty cheap, light and you can give it a lil' sanding and then spray paint it. Plus, it comes in super long increments, so you can basically cut it down to your perfect size!
I'm making the spearhead out of a couple of things. I'm using pink insulation foam, foamcore (posterboard with the foam sandwiched in between) and some plastic plates for the mini diamond (similar technique to my Skyward Sword Zelda
diamond. I made a digital sketch of what I wanted, printed it out to size and then cut out all my pieces. The hardest part so far was getting the edges sharp instead of rounded. If you'd like me to explain it in more detail, I'd be happy to, but I don't want to seem overwhelming! >__<
If you're not comfortable with pink insulation foam, you could probably make the same thing with just using really stiff posterboard and then gluing the edges together on slants. Or starting with a styrofoam base carved to the shape you want and then covering it with paperclay or paper mache.
Let me know if you'd like me to expand or anything!