I have no problem getting 6 x 2 channel ,not sure why you cant ,theres 32 lengths in stock at my local steel supplier today .What thickness are you going with on your crossers ?must be very thin if you cant find I beam of simular thickness .
How are you planning on mounting your deck boards ? if your plan is to screw them to the crossers ,you will be creating a way for water to enter your tubing .If your looking to install your clearance lights right in the tube ,you have created another area for water to enter .
Your correct in thinking trailer and truck body manufacturers get metal custom bent for them ,not all do ,and many are made with off-the-shelf standard dimension stuff as well .
I have worked on heavy trucks for 20 years,and owned my own shop for many years during that time ,and have mounted,repaired and modified many flat decks in that time .I never saw even one that was made with tubing ,not even the aluminum ones ,they were all made with channel ,angle and i beam .
Good luck with your project .