It depends on what I'm building. When I was building sandrails and race cars I was using Pro-Werks because I would purchase them at the same time as my 4130 chromoly tubing from Chassis Shop. I still buy them from there on occasion but most of the time McMaster Carr lately. For the bigger ones such as 1-1/4" and up I've used Ballistic Fab and have been pleased with the pricing and quality. If I don't have the time to wait I have had to machine some in a pinch as well.