EBay, my local welding supplies/steel merchant keep them in stock in black.
Dan
Try a local plastics supplier, most will be able to get it.
Failing that Barnes, I have used them for years.
I have never heard of these but the range is huge, and they seem to produce their own, but no round rod which is what is the most common.
I can check where we get ours from, I do not have any input in that area anymore, if you wish.