The Stopmotion Magazine tutorials are good. There is also a nice slow, detailed video on just making the mould of a body done by Ron Cole, in 3 10 minute parts. Part 1: https://www.youtube.com/watch?v=TDW1NKIfgQQ You should see links to part 2 and 3 in the "similar videos" column. This was a foam latex puppet, but the mouldmaking process is the same.
If you have never made a mould, start simple. Maybe try a 1 piece, open mould first, like you would use to make a face mask. Sculpt a face in clay on a board, pour plaster over it, let it set, pry it off and remove all the clay. You have an open mould with no back, perfect for a mask. You will get a feel for the materials that you can't pick up from just reading about it.
Then a smaller 2-piece mould, like maybe just a head. That will take you through all the stages, without the trickiness of a full body mould where you have to make your clay dividing wall around all the fingers. But it's the same really. If you make some mistakes, it's less work and materials ruined than if you did a big complicated mould first up. And everything will make sense when you have actually tried doing it.
If you are gong to cast in silicone, best choice for the mould is probably Ultracal 30, which is a very hard, fine quality plaster, that is light grey in colour. Or you can use Hydrostone, which is similar, but white. It's the one I usually use for the first 2 or 3 coats, but then I use a cheaper casting plaster with fibreglass matting to reinforce it on the outer layers. (Hey, the high quality bags are expensive by the time they ship to Australia, but the ordinary plasters are made domestically, so I use the good stuff where it counts, on the surface.) I've also used a couple of other better quality plasters, sometimes called "dental stone".
You can use epoxy resin, with woven strand fibreglass reinforcing. You can use polyester resin with chopped strand fibreglass matting, like I used on the Sumo wrestler mould. I think there are some hard urethane resins that also work well for moulds.
Here's a much quicker view of making the mould of a creature like that. (One of mine, for the indy film Dark Earth which is now in production.) It also shows the whole process including animating the finished foam latex puppet. Pretty much the same process as Ron Cole's, no detailed instructions but for a quick overview you won't have to watch for very long to see the mouldmaking part. Main difference is the head is part of the sculpt, not made separately like it was with my silicone Sumo guy or Ron's Rhinotaur.
I googled "Polyester resin Tulsa Ok" and got this Yellow pages link to suppliers near you.
It's a pretty common material, used for building and repairing boats and many other things, so it's not as hard to find as the specialist things like soft silicone for puppets and makeup effects.
Otherwise, you can order many of these materials online from sculpting and mouldmaking suppliers.
I found a little fiberglass kit at Amazon, but it was Amazon UK: https://www.amazon.co.uk/Polycraft-Fibreglass-Repair-Resin-Tools/dp...
You're better off using the stronger plaster (Ultracal 30, Hydrostone, etc.), at least for the first couple of coats, than ordinary plaster of paris. But it can be done. Use reinforcing in the plaster to help stop it from cracking - hessian (burlap) fabric, or chopped strand fibreglass matting are good. I've made hundreds of plaster moulds using Casting Plaster, mostly for casting liquid latex in, and it's pretty much a basic plaster of paris.
Mold Star 30 - I had to look that up, it's a moldmaking silicone from Smooth-on. It's ideal if you want to take a mould of a hard object, or cast a hard resin object from it, because it is flexible. From Smooth-On's description: "...good for casting wax, gypsum, resins, concrete and other materials." You don't need the mould to be flexible if your sculpture is plasticine, and your cast will be in soft silicone. It's not a problem that it can flex, just not a benefit. And with flexible moulds, you usually make a "case mould", also called a "mother mould" - a case around the outside of the silicone, to support it so it doesn't sag or distort out of shape. And what are the cases made from? Either plaster, or fibreglass and resin, usually, so you've still made something in one of those materials. The hardness of A 30 is good though, not too floppy. You will need a release agent to cast silicone in silicone. If Mold Star is a platinum silicone, the platsil or dragonskin could bond to it, without release agent. If it is a tin cure silicone, I believe it might stop the platsil curing, so it needs to be well sealed off with release agent to prevent that.
Found this on a message board -
USA -- Tulsa, OK
Locally, I am able to get UltraCal 30 from:
Independent Materials Co.
34 North Owasso Avenue
Tulsa, OK 74120-1628
It dates from 2011, but should still be good, although the price might have gone up a little. But hey, 50 lb of Ultracal costs $52.80 here in Australia, so it looks pretty good to me!
Why not just get some Ultracal 30 when you place your Smooth-on order? They carry good moldmaking supplies. You should be able to get just about everything you need there.