these boxes are sold on ebay all the time, just keep an eye out. Zenstate makes a good point, for cost reasons as well as availability reasons, just get a box and fill it with peanuts. But, if you can find some styrofoam from another item use that. An object doesn't slide around inside of a box near as much with styrofoam, compared to peanuts.
If you do decide to use peanuts, for your sake, don't forget to put a bag around the powermac. The bits and pieces of peanuts will be all inside every orifice on the system.
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"Fix it 'til it Breaks."
Jacob - EiC & Director of Technology Mac Pro Quad 2.66 - 4GB RAM 160 GB SATA RAID 1 - 650 GB Storage Quad 19" Widescreen LCDs Accessorized to the Hilt