The Silicone Plastique can be bought from online retailers as well as Amazon & Ebay. I bought a 200g tub for £14.95 from The Cake Decorating Company, and whilst it doesn't seem much 200g would make approx 25 of these small Lego brick moulds.
The Silicone Plastique comes in 2 parts which need to be mixed together to work with. My advice would be to only mix as much as you need as you can't re-use it once it has cured.
It really is that simple. You can then use the mould for chocolate, sugarpaste or clay to make cake decorations, keyrings or pieces of jewellery - the possibilites are endless.