C3PO Builders and TK-409' site are probably the top resources for Threepio. Chris even built a Threepio backpack, which is what I assume you are going to be building.
I've got a box full of Threepio parts (but no head) that I plan on turning into a pack at some point, probably in the fall.
Here is the C3PO costume we did.. Only 2 fully wearable made.
This was made slightly bigger than the original so a bit wider sizerange could fit it but it still in proportion:
We aquired allot of variouse parts from variouse sources as Disney and studio parts. Allot of the parts were not in to good shape and we also saw that making it proportioned we would need to sculpt allot totaly over. So it ended up by a total overhaul with allot of new sculpted parts.
The head comes from one of the Original heads that they test chromed. We aquired it from the Original chromers outside Elstree Studio after we tested our own chrome types.