I'd buy new to be honest .... the ballustrades I've seen seem to degrade quite fast.
I reckon it's to do with the fact that they are so full of twists and turns that if they aren't cast properly you could have areas of them badly filled etc.
Any place that supplies concrete products should stock them.
Be aware that they are a nightmare to paint .... my father has them in a patio area and it's the most hated job in the garden having to paint them!