Traditional quilting fabric is 100% patterned or plain cotton. Old and sentimental clothes are even more traditional. Everything goes, actually.
My "storytelling" quilts combine piecing and applique. I don't invest in sweat and take every shortcut to make quilts faster and easier -- machine piecing, quilting and detailing, for example. I don't need quilts for any practical purposes, and I doubt anybody does. Quilts are made to fulfill sentimental or creative need, or both. That said, I use quilting fabric as an expressive medium and I'm not very concerned with its durability. Quilting 100% cotton isn't the most durable fabric, anyway. Worsted wool would work far better, but it never comes in exciting patterns. Silk lasts long. I use old clothes and all sorts of samples and found pieces. Upholstery fabric samples are great for quilting, because you can get the families of related colors and patterns. They are hard to work with: thick and fraying, and don't press well. Second hand linens from thrift shops are great for backing: they come in beautiful colors and as large as largest bed cover quilt.
Speaking about storytelling and expressive use of quilting fabric: when I look at fabric, I decide what story it belongs too. Not literally, but in some subtle way. I hate thematic patterns, like snitches for Harry Potter theme quilt, or blue and white khanukkiah for everything Jewish, or pumpkins-on-black for Halloween. These crutches for fantasy are insulting and very limiting. They can possibly work in small quantities within very complex geometric pattern, but not in a figurative quilt. I mean something different, like "this greenish sickly blue knit is good for a trouble at sea" -- a pirate story, or a whale-swallowed-a-sailor story. Or, this uneven pink and green striped silk look like fish in sun-lit pond.
Fabric stores have special quilting sections with quilting fabric. Quilting fabric is mostly 100% cotton, sometimes cotton/polyester blends, 45" wide (60" for some broadcloth). Jo-Ann and Hancock stores sell quilting fabric starting at $5/yard, but some nice artsy patterns can sell at $13/yard, maybe more; I'm not sure since buying expensive fabric to cut it and sew back together starts to seem absurd at some point. Plain broadcloth is sometimes thinner than patterned fabric.
See also: quilting supplies, sewing machine.