Driveway markers at canadian tire. The black one not the orange ones. they are the same diameter just cut them to size and sand the cut so you dont have sharp edges poke through the corner.
update: I searched the ctire site and could not find the black ones, may have been replaced by high vis orange. I bought 2 last year on clearance for $2 just in case and one of my poles splintered slightly last week when I was folding it up. The ones I bought are just a black fiberglass pole with a metal tip to drive into the ground with no reflector attached. It has a couple reflective white stripes on the top.
here is a link to an orange one but just go in and they may have black as well. They have many different models, just look for a thicker pole to match the one you have.
http://www.canadiantire.ca/en/pdp/certi ... 3313p.html#.VL0XgbM8K9I