The straight dump rams horns are also available as a standard replacement part from Dorman. I think you can get them at any PartsAmerica.
Some GM dealers also still sell them (Gilbert Chevrolet used to sell them over the web), but I think they are old stock.
Link to the dorman manifold
Based on the pictures, the 2 1/2" are straight dump, while the smaller ones from the trucks are angled dump.