If you want cotton, and in larger quantities so that you will be able to quilt with it, I buy mine from Empress Mills in the UK.
They do 100% cotton (50s), 1000 m bobbin for £3.00 and 4000m cone for £9.50 in about 90 shades. Quote from their catalogue " This fine strong, soft finish natural thread has proved itself as a winner, particularly with machine quilters - super sewability- a sell out at the Shows!"
They are a family owned company, and I've found them very helpful.
www/empressmillls.co.uk (no affiliation, but I am busy building up cones of basics for my collection. I used about half a large cone quilting a king sized quilt recently, with lots of feathers etc.)