Help please.  I am inspired to get a larger, quality, weather resistant canvas tarp to make a primitive outdoor shelter.  

It would be used to cover a common area near tents on a bowhunting trip or a little shelter at deer camp or in back yard to primitive camp with young son.

Thinking at least 12x18 maybe to 14x20.  Used to make a basic shleter like middle drapped over a horizontal pole and sides spikes to ground or over a high horizonal pole with sides tied off to trees.  Not looking for a whole wall tent

What is best source for the tarp?  Any specs that are must have?  Thanks.

I think this would be useful in bow hunting camp.

Take a look at Panther Primitives.   Their canvas is high quality, lightweight, and treated.   You can buy canvas by the yard and make your own.   Or you could just buy a wall tent fly that is ready to go with sewn seams, loops all the way around for the side poles,  etc.   Their catalog is a wealth of information that will give you lots of ideas.