Most of the mining machinery I've worked with (up to 100 tonne) has had hydraulic retarders and I think a lot of buses do too (they certainly sound the same). I suspect hydraulic retarders are preferred for low speed, repeat applications as they're more effective to a lower speed and easier to dissipate the heat (giant radiator). Also you don't have to generate electricity to operate the electromagnets. Once you get into the electric drive stuff then they turn the motors into generators and dissipate the electricity through resistor grids.
I think you'll be more likely to find electric retarders on vehicles that operate at highway speed. From memory when I was trying to find one the main trucks that have them is Scanias and even then only occasionally. Unfortunately second hand Scania gear is expensive and thin on the ground.