You can use just about anything…as Jim mentioned PVC gets used a lot. If you want to make it bomb proof run 3 sets of lines at 130 degree angles around the pole. If each set contains a line at both the middle and the top of the pole you should be golden. To get an idea of what I am taling about go to the 2006 album and click on the Garfield photos to give you an idea. If you have supporting ropes at the top as well as the middle then you will get all your stress on the rope and not the pole itself.
You may want to consider electrical conduit. We have been using a pole of 1-1/4 and 1″ conduit with couplings to hold together. I think it may be more available (i.e. giveaway from somewhere) than PVC, which by the way is not cheap nor is it light weight. Ours is cut into 3-foot lengths (aka scraps) and 20-feet total is 18 pounds. I can send you some pics if you’d like to see. You could also check some of the pics using the links in my signature. Also parachute cord from EMS is our guy rope of choice. Always sturdy with the 6×10 flag of ours.
I have a flag and pole that I put together for 2003 that you can borrow. The pole is made of 2 sections of PVC, 5 feet each, with threaded coupling to connect them. Check pics for Eisenhower 2007, 2005, 2003.