I have always carved my moulds into soapstone for casting white metal, the moulds generally work best when they are two part with a pouring hole, and when the moulds are pre heated to at least quite warm, I use a candle to deposit thin layer of soot inside the mould (this helps air escape and acts as a release agent) sometimes you need to cut additional air channels to help the metal flow, usually the first few mouldings are rubbish until the mould heats up, just throw the metal back into the pot and re-melt.
my source of soapstone was to find a shop selling soapstone figures from africa etc, then go in and ask if they have any breakages in transport, usually you can get them for nothing or at least very cheap. (crocodiles cut in half lengthwise are good for long moulds) you can do open topped moulds, but the backs may need sanding to make them flat.
really its about trial and error, and being prepared to make mistakes - at least with white metal its fairly forgiving, one source for metal is old lead free pewter tankards, you can pick them up at jumble sales and boot fairs dirt cheap if you take the dented ones.
Grendel