Flat poles as ground rails are much safer than round ones. You can get some pretty nasty (and in a worst case scenario career-ending) injuries from horses standing on round poles.
Scaffold planks cut in half length ways make perfect ground poles. They are sturdy, designed to hold weight and withstand abuse, and most importantly, cheap.
There are two possible sources for these – either befriend a local scaffold firm, because they have to replace them as soon as they show any damage, and usually just chuck them out or burn them. Alternatively, do what I did and find your nearest Wood Recycling Project (you can find links to them all here). Ours sells scaffold planks for £1 per foot, meaning I got six good 12ft rails for under £40.
They are ideal as trotting poles, and as ground rails in gridwork.