I have used Threadart's own
http://www.threadart.com/shop/category.aspx?catid=246 for my Free motion and other quilting needs for over 4 years and I find it excellent and for me in the UK, even with Postage, their prices are unbeatable, as is their quality.
They have a Sale on at present so I am awaiting a delivery of an order just now :thumbup: