I am building a trebuchet and looking for potential sources of weight for the counter-weight?

Scrap steel is around $0.15 /lb and your concrete is $0.04 per pound so steel will cost more assuming you buy it. But the cost may be worth it for the added density. Go to a slavage yard and look around. Railroad track, I beam, electric motor cores, etc may be a shape you can use. Steel chain molds to a shape or wraps around a beam.