I use R & M Rug wool, a really heavy cabled wool, almost as fat as rya, but firmer. I have also used wool from the fiber studio. Theirs is a really firm two or three ply twist, that will not shrink! I have used it successfully, also, and depending on the look you are going for, you might want to try both.
http://www.rmyarns.com has a on line look at the yarns, and http://www.fiberstudio.com will let you check those out.
Cathy in W. Tennessee
Consider Robin & Russ, in McMinnville, Oregon (they do have a variety of linen and cotton warp materials)
On Rug Wools I have used Halcyon 3 ply rug yarn with good success on my seats also Weaving Southwest's tapestry and rug yarns. I like the 2 ply tapestry yarns as they are finer and I then ply it as I weave to a thicker yarn. This lets me increase the number of colors as I grade from region to another. I only use Weaving Southwest's Navaho wool rug warp. It much more elastic than linen and have never broken a warp thread when using it while I have had linen 'shatter' almost chaoticly.
I have never seen Webs mentioned as a source for Linen Rug warp. They have the natural in all sizes 8/6, 8/5, 8/4 and 8/2. It is Irish linen and seems to be very good. They use it at Harrisville for their rug classes. Webs offers discounts to anyone if you purchase over $60 (20%) and $120 (25%). I have been using them for ~12 years with satisfaction. They can be found at http://www.yarn.com
Tim in east TN. (I have no affiliation)