I got one this weekend ($197) from a computer fair in the L.A. area.
I went there because I wanted to visually see that it was a C1 for myself. Out of 10 booths with P4's that I checked, two had C1's.
Here's the phone # of the place I bought it from if you want to try--TCC Computers 626-642-1000. When I told them why I was looking at the codes, they were oblivious. I certainly can't vouch for their whole stock being C1, and personally I wouldn't trust a specific purchase like this over the phone. Not that they might not be honest, but just because of a possible language barrier and their lack of understanding of why it's a big deal.
I saw B0's that had packaging dates more recent than mine (10/7/02). My point on this, is that a retailers latest stock is not guaranteed to be C1.
I would absolutely trust PCNut if they say it is a C1. They are a great outfit. But their prices are kinda high.
I haven't heard anything good about atacom.