When our 18th century foremothers cut out their shifts, they were doing so on 3/4 or 7/8 yard wide linen - in other words 27" or 31 1/2" wide material. Today our linen fabric is 54" or 60" wide. So isn't wider better? Not when it comes to laying out fabric for a shift. Why? In one word: SELVAGE!
When making a shift, a good selvage means there's no need to finish an edge of a seam because it won't unravel during washing and wearing, so the more edges that fall along selvages the better off you are - less seams to flat fell. The linen widths sold in the 18th century allowed the maker to to maximize her selvages and not end up with that crazy mish-mash you get when you try flat felling all the seams that end up at that nexus at the top of the gores and side seam -- the bugaboo of every modern shift maker!
Here's an advertisement typical of ads for linen in the 18th century. This one is from the Massachusetts Spy from June of 1773, advertising 3/4, 7/8 and yard wide linen.
So why can't we get linen in those widths? Because fabric in the 18th century was woven specifically for how it was going to be used. It's that way today too, but nowadays most people aren't using linen to make shifts -- linen is primarily used for home decoration where wider is better - think curtains, bed spreads, furniture, etc. Plus it's more efficient to weave wider, which is another reason our linen isn't as good, but that's a discussion for another day. So if you are lucky enough to find a good piece of 3/4 or 7/8 wide linen with a decent selvage - do not pass go, do not collect $200, buy it!
N.B. Sometimes you can find vintage sheets in narrower widths. If they are not too old and not too heavy, they will work. However, sometimes they are just plain old and you end up with a shift that you've spent all kinds of time to make, only to end up with holes in it after a few washings, so be careful when buying vintage textiles for shifts.