Any of the oil based clays can be melted like that (I remember the post you're talking about). I have always used a brand called Roma Plastilina, but they are all basically the same. You can't usually find them at common retailers, like Walmart, but might be able to at Michael's or other craft stores (I couldn't). The best prices are found online. It runs $6-10 for a 2lb block, regardless of brand / hardness / etc.
I will tell you that, in my opinion, what that guy did by melting the clay and pouring it into the paper model was not necessary. He did it because he is afraid of fiberglass resin fumes (I think he said he had lung problems). It would be much easier to resin / fiberglass the paper model, and put clay on the OUTSIDE for details. He went through a lot of work just to avoid the resin.
Depending on which method you decide on following, you'll need several blocks of it. I used about 15 blocks (30lb, $120) for making my armor. If you fiberglass the paper model, and just use small amounts of clay on the outside, 5 blocks would probably be plenty.
One final note, especially if cost is a concern, melting the clay like that guy did, RUINS IT. It makes it drier, and it cracks easier and becomes more brittle, which is exactly what clay is not supposed to be like. I tried doing something similar to him, by melting it, when I did my armor, and I noticed that it was ruining the clay. If you find that post, you'll notice that I asked him about it, and he too said it was not good for his clay. It's just sad to throw out $100 worth of clay, but it was ok to him because he wanted to avoid fiberglass resin.
