Hello, I just picked up my aluminum timing cover and water pump from Summit Racing, but they are often easily found used. You could try craigslist or other classifieds.
IMO if they need replacing, I'd prefer aluminum, which dissipates heat better and offers both weight savings and potentially a performance increase. That also depends on what you want out of it. There are high-flow water pumps in both cast iron or aluminum that offer improved cooling. I had a tuff-stuff brand high flow cast iron pump in my 289 that worked great.
I would avoid a used water pump just because there's always a risk of premature failure, and the cost difference would more than justify the additional expense for another one if it ever went out. Used timing covers can also have warps, cracks, or stripped threads that are important for proper water pump sealing. I'd also recommend checking national parts depot for some decent prices on new stuff.