You can buy really good tight weave canvas from Utrecht Art supplies
As a matter of fact the folks that make tents from linen canvas buy the linen canvas (unprimed) from Utrecht, but to make a military wedge tent is way more expensive than buying a canvas one already made, and..., it's not fire retardant, which more and more historic sites and insurance companies want all of the tents to have fire retardant added. (reenactment units are often required to have unit insurance) Most of the after market retardants are water based so not suited for outdoor shelters.
LD