Fabric for Pants no. 2 needs at least a 5% spandex content. Cotton-spandex blends work very well and companies like Cotton + Steel, Laguna Jersey, Andover fabrics, Art Gallery Fabrics (AGF), and BOLT by Girl Charlee offer several prints and solids.
Stores like Fancy Tiger Crafts, Gather Here, Hart's Fabric and Stonemountain & Daughter organize fabric suitable for apparel and have specific sections for knits. Read the fabric description, for materials and suggestions. When in doubt, call or email the store to ask whether the fabric you're interested in would be suitable for leggings.
What to avoid: 100% cotton, interlock, sweater knits and linen blends.