The Compleat Sculptor, Inc. (Sculpt.com)
More specifically: http://sculpt.com/catalog_98/stones/SoapStones.htm
The Italian Green (which I like a lot), comes in block form, in sizes from 2-10lbs.
I just weighed a mold that I did from that same kind of stone (the "way of the sword" token), which I think is on the medium side when it comes to molds, is 8 oz on the nose.
I was very happy with my sculpt.com order, they shipped me nearly perfectly rectangular blocks (20 pounds of Italian Green, in 3 to 4 pound blocks, half of which I used for a class). Call them though, there is no space to comment on the online order form, and they will delay shipping and contact you for clarification if you order an amount larger than they have in stock as a single piece.
The Complete Sculptor sells a variety of different types of soapstones. The pricing on them seems very reasonable, but they often ship very rough shapes, not neat squared off blocks. Aside from the increased prep time, if you like neat square carving blocks and molds, it will greatly increase waste.
Iíve tried several types of stone from them.
Their Pink Talc is a very soft, easy to carve stone. Its light color makes marking on it easy. While nice pieces can be a dream to carve, Iíve run into pieces that had a number of small cracks or tended to crumble a bit much. I donít order this anymore.
They also offer a stone called African Wonderstone. This is significantly harder/denser than any other soapstone Iíve worked with. (Not including counter top cut-off scraps, which I donít consider suitable for carving.) African Wonderstone is what one of the professional pewter casters Iíve spoken with swears by, because his molds last longer, but it is also quite a bit more difficult to carve. It is also a much darker stone making markings on the stone harder to see.
The few pieces of Brazilian Green Iíve worked with have carved very well. It is a soft stone, but not excessively so. But it is almost translucent/glossy which can make seeing exactly what youíve done a bit more difficult.
They also offer several additional types that Iíve had minimal experience with.
Fair warning: Iíve occasionally had a hard time dealing with these folks. Iíve faced long (unexplained) shipping delays, billing mix ups (theyíve lost my credit card number at least twice after I ordered), and had trouble getting information about my orders from them. Everything has always been resolved to my satisfaction, but its been a pain.