Pieces of PVC piping would probably work best for you if you're flying with them. That way you can have a few 2-3 ft sections that can be connected together. That's what Pea used last year for his and it worked out well. He didn't build his very tall though as we just duct taped it to the top of a friend's RV. I'm not sure how stable PVC would be on it's own, but if you have something to attach it to, I think that could work well.
I had a scheme to pick up a small helium tank, some heavy duty balloons, and basically 3 or 4 of those thin rubber coated steel dog leashes (kinda like a bike lock).
The plan was to attach it to the vehicle (and lock it to ward off thieves), blow up the balloons right in the campground, and fly that as a flag. There was also talk about attaching a solar powered strobe light...
you could easily pick up all of these things at Walmart on the way, except maybe the strobe.
Yea PVC pipe is really the best way to go. Cheap, easy, and can be broken down into small sections. I lucked out having an RV ladder railing to duct tape mine to last year though, because otherwise it would have only stood about 5 feet off the ground. Wtf was I thinking?
I should have a couple different RV duct taping options again this year, but I think I'm going to still construct an even taller flag pole since we'll be having more people trying to find our campsite this year. Just make sure if you end up going the PVC route that you bring PLENTY of extra duct tape to wrap around each joint. Half of my flag pole got blown away in the wind last year because the top section got rocked out of one of the joint sections.
I think when I was looking into it for Pemberton there were mini helium tanks going for around $30 from Walmart, and I figured I'd go to a specialty balloon store to try to find two or three HUGE balloons. Probably wouldn't take too much really.
I've always used dollar store bamboo for flagpoles. They don't have joints like PVC, so you have to double them over each other a bit and wrap the overlapping sections in twine and/or duct tape. I always buy the thickest and longest sticks available, usually at least a meter long each. They are cheap and therefore disposable so I never care if they make the return trip.
However you do it, you definitely want to fly your flag(s) as high as possible. And your flag(s) should be as unique as possible, too. Otherwise you might be surprised to see how many similar flags are towering over yours when you're trying to find your site.