Yes, Thule sells the cross bars seperate from anything else. They come in different lengths - I bought the longest available (about $10/ea more than next size down) and cut them to size. You really dont need the Thule ones though - in fact, I might have preferred to have just bought some good grade steel tubing as I dont like how the Hullivator mounts "catch" the Thule cross bars. The mounts have a metal strap that doesn't quite catch the bars too well.
In this picture, the metal strap (E) slides down the mounting tab in the base arm (A) and then swings under the cross bar and is bolted to the far side. In designing this strap they made a bevel in it (i'd imagine to better fit round bars - but from other posts it doesn't do that too well).
This allows the base arm (A) to slightly twist on the cross bar - not too noticable when just mounting the arm, but hike up an AI and you'll see what I mean. I plan on using some steel plate material wider than the cross bars to take out the play - hence the thinking that properly sized steel tubing used as cross bars may eliminate that need and shore up the whole rig.
As to 7 feet, I'll have to do some measuring later on as my rig is now dismounted, but I'm 5'7" tall and have a tall SUV, Im at the upper limits with my arms fully stretched and on toes ... which I just measured at - drum roll - exactly 7' !!
Also note, any steel tubing MUST be protected with a good couple of coats of paint or better yet, powder coating - triple so if your useage includes salt water.