You can buy seamless stainless tubing at metal suppliers but it costs big $$. Well it cost big $$ compared to steel brake line anyway.
You also want to spend a bit of extra money on quality flaring tools. Dont go with the cheap ones or you will very likely regret it by the time you are done. If you decide to go with a 37 degree flare be sure to cut the tubing with a saw or dremel of some sort. The tubing cutters supplied with most kits will deform the tip of the tubing a bit making it more difficult to make a clean flare without lots of deburring. It doesnt matter quite so much on the 45 degreee as you will be folding this part under anyway with the double flare.
Blaine