I have found a couple of additional sources:

http://www.najecki.com/repro/Linen.html

which appears to have a number of good choices (both flax and hemp
linens) but from whom I have not purchased any fabric, and

http://www.denverfabrics.com/

where I purchased 3 yds of very nice 45" wide 100% linen that was
labeled as medium-heavy natural linen and turned out to be 8.7 oz/sq
yd (Stock # EE565 which no longer shows up when I search for it on
their web site so it may have been a clearance item). They have other
100% linen fabrics (though not natural color) that are listed as heavy
weight at reasonable prices. Denver Fabrics is actually in St Louis at
10490 Baur Blvd.

Doc