Maybe you have a class in school where you used water based clay to make pots or something. This is not the same type of clay that you will want to use to make armor, not ideally.
What you should use is oil based clay, sulphur free if possible. It is widely available from sculpture supply companies and art stores. The brand Chavant is known as a very good brand of clay. Do a google search on that name, and you should find plenty of suppliers.
If ABSOLUTELY necessary, like if you live in a hole in the ground you could do this with water based clay, but it is really not preferable. It will take more time, and effort to produce results this way...
bro...just my two sense. i'd say to use a chavant clay. personnolly id go with the idustrial CM-50. its a very dense de-aired hard clay (used often to make prototypes of cars and trucks). or if that doesnt sound too apealin have a look for yourself http://www.chavant.com/new_site/files/pdf/products-2006.pdf <--theres the product catalog. good luck
If your looking for cheap-ish clay that can be used for moldmaking, try plasticine clay. At the local dollar tree, they sell little packets of four 100gram bars for $1. Might want to check one near you to see if they have any.