That 17mm size appears rare.

http://www.chiefaircraft.com/radio-c...tab-tubes.html

http://www.troybuiltmodels.com/categ...par-Tubes.html

Also - it doesn't necessarily have to be a tube.  You can make an "X" cross-section out of two lengths of aluminum angle iron (overlap the angle just enough to add a fastener every six inches or so) or even slotted aircraft plywood that is 17mm across the flats and epoxied along the joints.