RockAuto has the Spectra Premium at 1", and a few others at 7/8". You can also look at the 2004 Regal's, which list a few 1" ones (bought my replacement one there in 2009).
Since you're replacing, might as well get the nicest option. As mentioned, a radiator isn't a necessary upgrade, unless you're running many other items adding heat in the engine taxing the cooling system more. I only started upgrading when intercoolers, turbos, lower/drilled t-stat, and so on would cause more heat, work the system more, and reduce airflow over the radiator.
Not as simple, but you can also shop for some of the pre-97 GP's that came with 1" to 1-1/4" or so (ex. 1994 3.1L or 1991 3.4L GP list some), and adapt them to work (fan mounts and trans fittings changed by 2000). I got my GTP's 1-1/4" from a junkyard early-90's GP that must've had it recently replaced.
