Before I became a weaver I always thought, yarn is yarn. I've always had a stash of stretchy, buttery soft yarn stashed away in my closet from my years as a crocheter and thought I was set! My first couple of weaves were fine on the loom, but as soon as I released the tension they would shrink and loosen and get uneven bubbles... and I soon discovered that weaving yarn is a species of its own.
The best kind of yarn for weaving is denser cotton and wool with a bite to it. It can't be too smooth or it will slide out of your weft and it can't be too stretchy or your tension will be a nightmare. If you can find it handspun and hand dyed... that is the stuff dreams are made of!
Here is a list of my favorite places to find yarn especially for weaving: