You don't have to use just wool. You can use anything you want and combine these yarns any way you want. Wool is the easiest to weave with because it is so elastic. Silk, cotton and linen will make your life a little more difficult because they don't have any give. Problems with pulled in selvedges will happen more frequently with these yarns.
Don't forget the concept of weft blending: taking two or more yarns that can either be of the same fiber or different fibers and using them bundled together. This can product lovely shading effects. For example: using a strand of silk or rayon combined with wool. The silk or rayon reflects light and the wool absorbs it making for an interesting effect.
But let's get technical for a moment and I will explain what really makes a great tapestry yarn. The answer is long, glowing fibers from certain breeds of sheep that have been combed (not carded) in preparation for spinning. Combing fibers aligns them whereas carding fibers gets them more tangled up. Yarns made from combed fibers served two purposes: they were used as warp because of their strength and they were used for outerwear, blankets, saddle blankets, rugs, etc because of their durability. Carded fibers are loftier and are used for sweaters, socks and other wearables that would be closer to your skin. For example, merino, which is a very fine, short fiber is great for sweaters but would be lousy for tapestry.
Who are these sheep that provide great fleeces for spinning tapestry yarn? Here are some of my favorities:
The following is a compilation of tapestry yarn sources. I am stealing most descriptions directly from websites where these yarns are available.
From Fine Fiber Press: http://members.peak.org/~spark/FineFiberPressTapestryYarns.htm
ALV (Elf) Tapestry Yarn
This Norwegian tapestry yarn is a 2-ply worsted yarn made from 100% combed long fiber wool. It works beautifully for woven tapestry. Kathe uses four strands together, with a sett of 10 warps per inch. There are 700 meters in 100 grams (Approx. 765 yards / 3.5 ounces or 218.5 yards per ounce). NOTE: We sell it only in units of one or more ounces!!
The cost $4.00 per ounce plus cone price of 25 cents. If you provide the paper cone, we will take off the 25 cents. Inquire about cone size. Our winding machine will only use a larger style cone such as those used for standard weaving yarns, not the smaller ones that usually came with the Australian tapestry yarns.
Here is a view of the colors. However, computer monitors vary greatly so we suggest that you order a yarn card and base your color choices on that card instead of your monitor. The color card costs $3.00. NOTE: The color card is actually made from this company's 3-ply yarn which is a larger size than the 2-ply one we sell. The colors are the same however. They include a sample of the 2-ply yarn in the lower right hand corner of the color card. The title of the sample says: ALV Kval. prove which means ELF sample.
These are the colors!
From Weaving Southwest: http://www.weavingsouthwest.com/shop/product/104
Hand-dyed Tapestry Yarn
4 oz. skeins
approx. 162 yards.
Brown Sheep Yarn Lamb's Pride and Top of the Lamb: available in lots of places and is used in our ipod kits
Although intended for knitting, this yarn comes in a lot of gorgeous colors and works very well for tapestry. It's available in pure wool (Top of the Lamb) and in a 85% wool/15% mohair blend (Lamb's pride). It comes in solid colors and painted colors. Below is an example of a few of their colors. Who knows what fleece goes into this yarn. What is "wool.?"
Borgs Yarn from Sweden: http://www.vavstuga.com/store/wool.shtml
Claudia's notes: I first discovered this yarn at The Handweaver's Guild of America's Convergence. There was a sale bin full of it. I bought enough to fill my suitcase. I was in heaven. It used to be available at Unicorn Books and Crafts, but no longer. This is a worsted weight yarn you would use at a 6, 7 or 8 warp sett. My research found the above link.