Large quantity of modeling clay.
 

I've purchased the close out of a modeling company. I have 500lbs of clay. I will bring to meets if anybody is interested, PM me. $2 per pound, usually cost is $4-6 depending on qty ordered.

This clay is reusable if you keep it off of the floor. I used some larger baker's trays under the area where I would scrape excess from my model. Just wad the scrapings together, rewarm to 115 degrees in a medium large toaster oven and apply as needed.

This best way is to get a relatively close shape with foam or other cheap material, then skin coat with the clay to get the exact shape you are after. Because you can add, remove and add till you like what you see the clay is ideal for developing/ creating a look.