I think some 02s had them but it was mostly 03+ Bonnevilles. I dont know what other cars will interchange.
If you just need the CD player portion i can get you that for cheap, if you need the amp portion I'd have to look around.
car-part.com will be your friend, look for "uz8" that is the option code for the monsoon amp.
2004 Radio Pontiac Bonneville GPX,5-04 AMP M0882 $125 M and M Auto Parts, 1-800-586-5492
2005 Radio Pontiac Bonneville GXP,4.6L,GRY,AMP,UZ8 6G5562 $100 AAA Auto Salvage - 1-800-238-6664
2003 Radio Pontiac Bonneville AMP 6A1615 $100 Sun Auto and Salvage USA-ME(Norridgewock) 1-800-843-5176
2003 Radio Pontiac Bonneville amp 'UZ8- 3768 $85.36 Hardy'* Auto Parts LLC USA-NY(Bernhard-Bay) 1-800-404-2880
2005 Radio Pontiac Bonneville AMP,OPT-UZ8; model#09356612 A 50078E $75 Stricker Auto Parts USA-OH(Batavia) 1-800-582-5405
but make sure it is the amp before you blow $75+ on a new amp