These come in various lengths and thicknesses. They're not as strong as rebar but they are cheaper and they worked well for me. I got them at clearance prices at Menards up here but I see them in several other places too.
Here's a link to Lowes: http://www.lowes.com/pd_93169-16418...ue&productId=3574108&rpp=32&searchQueryType=1
One other idea I wanted to try is the fiberglass marker poles that some people place at the end of their driveway. They're typically florescent orange but who cares if they're inside of a blowmold. These also seem to be relatively strong and there’s no chance of rusting.