Clothing is fine to use for quilting. Some prefer to just use cotton clothing. Many use sheets, dust ruffles , where ever they can find useable fabric. One note just make sure they all play well together in the washer... in other words all have the same washing instructions.
Sometimes just letting people know you are looking for fabric can help. I have plenty of my own fabric but people often give me fabric they have no use for .
One of my first quilts was old blue jeans .. cut up into squares... it was before rotary cutters ... and local quilt shops.