Half round you can buy easily enough.
On the tender above Fowlers used a flatter profile more like the steel you see on some handrails/low fencing. I just use 1/4" flat stock clamped either end to a bit of iron and filed the profile, did not take long and I only needed about 3ft total.
If you don't fancy filing have a look at this page which shows a way to do it my machine. though may get a bit whippy if you only want 1/8 beading