Cotton is recommended for quilts because polyester cuts the fibers of your quilt over time. Even if you purchased cotton-wrapped polyester, I've read where the outer covering of cotton wears down over time, exposing the polyester core which then works like a saw on the cotton fibers of your quilt.
I like Presencia 50-weight, King Tut 40-weight (they have a wonderful selection of variegated) and 30-weight Sulky for machine quilting, all of which are 100 percent cotton.
I use Presencia 60-weight for piecing.
The Presencia brand is a 3-ply thread, both 50- and 60-weight, whereas brands like Mettler are only 2-ply (not as strong). All or some of these brands should be available at your local quilt store. If not, there are plenty of online sources.