What's the stuff dentists use to make molds of your mouth?

What is the rubbery stuff that dentists use to make moulds of the inside of your mouth? I need to know what it's called and if possible where I can get some...
...It's for model making.

AS A DENTIST I CAN ANSWER THIS EASILY.
THE MOST COMMON IS A POWDER YOU MIX WITH WATER IT'S CALLED ALGINATE.
THE OLD ORIGINAL VERSION SOMETINES USED TODAY IS CALLED RUBBER BASE COMPOUND.
A MORE MODERN AND EXPENSIVE FORM IS IN AUTO-MIX CARTRIDGE AND CAN BE OF SEVERALVARIETIES AND PROPERTIES.
FOR MAKING MODELS ALGINATE IS A COST-EFFECTIVE OPTION. AS THIS MATERIAL DOESN'T LAST INDEFINITELY, RUBBER BASE IS THE ALTERNATIVE. HOWEVER, FOR MAKING MODELS OF THE MOUTH DENTAL TRAYS WHICH HOLD THE IMPRESSION MATERIAL ARE NEEDED.
TO GET SUCH TRAYS THEY NEED TO PROPERLY FIT YOUR MOUTH. ASK IF YOUR DENTIST IS WILLING TO LET YOU HAVE A FEW.

The dental version will be very expensive.

Try the model/crafts/hobbies shop, I have seen this for making moulds for plaster of paris.

Plasticine or cement

It's dental moulding. It's a powder that they put some chemical in and then mix with water to the right consistency. If you're after some for model making, I would suggest going to a craft shop as they will have products that are made for such a purpose. I doubt you can get products like this online or from a shop.

The most common material used to take impressions of the mouth is called ALGINATE, it's in a powder form which you mix together with water.
Not sure where you can actually get it yourself to be honest, why not try asking at your local dental surgery if they may be kind enough to let you have a sample of theirs!!!

The simplest material is alginate, about USD$15 per pound. Can only be used once before it dries out and distorts. Reasonably accurate.
Next best is vinyl poly siloxane (VPS), about USD $23 per pound. Can be repeatedly reused, very accurate.
Slightly better again is polyether, but more expensive again.
I'd suggest a model shop would have some cheaper suggestions, or if you really want some dental impression materials, then a dental lab would have cheaper stuff than dental surgeries.