I keep breaking my ArduCopter arches, so rather than order replacements, I just laser cut my own through Ponoko. I've uploaded the design, so you can do it too. Select 3mm clear acrylic, P1 size. It should cost $8.20 plus shipping for the four arches shown above. Note that these are laser cut, not CNC milled, so they don't have the notch on the side that fits snugly on the arms. But they work fine all the same.
Thanks to Sandro Benigno, you can also order a set of five replacement dome rings, as shown here:
All the other parts are in the ArduCopter repository, but need to be converted to SVG format to laser cut. If you see something you'd like to make, let me know and I'll convert and upload it to Ponoko. Most can be laser cut as easily as CNC milled, but in some cases you might need to do a little finishing work, like counter-sinking some holes. No biggie...
I've added the Main Square:
And the Legs:
And the carrier board: