GM did not make a bonneville with rear disc brakes til 2000, but they do not fit the 92-99 body style. many members have asked this question, and the general response is it is not worth it. The rear drums are more than adequate for stopping the car, and if properly adjusted with good front rotors and pads, they perform excellent.
That being said, I believe a couple people are still looking into options, and I believe one has attempted and succeeded, but I believe it was on an earlier model. Used the search feature and check out the results.
2007 Saturn Aura XR | 1970 Chevy Blazer 4x4 | 1970 GMC 3/4 Suburban 4x4