The best suggestion I can give is to get a tape measure, measure the inside and outside dimensions on the length of your bumper, the height, and how deep they are, note if it has any specific bends to it, for headlight clearance, ec. If it does, then get some measurements of these peculiarities, then go out to the salvage yard and find ones that fit your measurements.
If the mounting holes dont line up, you can either redrill the bumper (not pretty), or redrill the mounts. You may well have to do some fabrication to get a set to mount anyway, due to differences in frame widths, mount locations etc.
Not being a Buick man, I dont know if they made chrome bumpers for your car that year or not. It could be that they did for a model other than your Century. You might also take a look at other cars built on the same frame as your Century; Chevies, Pontiacs, Olds.