I might have a few tips for you.
There is a company here in Australia called Bodybitz that do the type of casting you are talking about. http://www.bodybitz.com.au
They sell kits, but I don't think they have one for just the female genitalia, though if you look at there website you will see they have cast plenty. They even have a pussy clock for sale.
I have seen them at some expos here. All they use is a little tray like container to hold the dental alginate between the legs.
If you know what your doing you don't really need the kits anyway. Essentially the kits have a container to hold the aglinate in and dip the body part into, some fast setting dental type alginate powder and some plaster powder.
So if you can get dental alginate and some plaster you should be fine. Have some imagination with the containers. Plastic softdrink bottles for a mans bits, round 20 Litre plastic laundry tubs for butts and boobs. Just vist the some local stores and look at all the different plastic containers, buckets etc. You should be able to find one for just about any bit.
I have used this method to cast some butts and even my back. There are some pictures of the casts and the moulds on my web site, www.bimmortal.com.
I don't like the dipping method to mould boobs, because I think it distorts the shape having them hang. For this I prefer to use the slower setting alginates. You need to be able to mix it thick so it stays on the model and still have anough time to spread it over the body. I have used the slow setting aglinate from ArtMolds with good results, and have just recieved some of their fibregel which I am looking forward to using.
Hope this helps