i buy alot of thread from Connecting threads- i have a long-arm & cotton thread tends to break every couple inches-along with creating a ton of lint (& not just from connecting threads- any cotton thread) so
for long-arm quilting polyester threads are more commonly used. i do use their cotton thread for piecing- they have a good selection of colors and the prices are good. i also purchase thread from Superior threads & Aurafil from Fabric.com.
I've been buying from Connecting Threads for a couple years & have never been disappointed with any purchase.