I am using 3/8 stainless...i got my 4' lengths on ebay. I also am using swagelok fittings....they are pricey but do not require a flare to seal which is why i went with them. I also got those off of ebay. seems to be the most cost effective way if you map out your system and buy a lot of surplus fittings from a supplier on ebay...one fitting can run upwards of $20 if you buy them individually. Also i got a bender from imperial that runs abut $60...the swagelok bender was over $200...this bender works beautifully.
http://www.ebay.com/itm/Imperial-Stride-Tool-Tube-Bender-3-8-Superior-quality-/290452078210?pt=BI_Pipe_Benders&hash=item43a04afe82