The grand prixs from 97-08 all use the same thermostat, NA, Supercharged, 5.3L GXPs, all use the same 195* thermostat. Figuring when it opens it will still run a little hotter, most will probably run about 200-210* depending on the car and outside temps and driving conditions. GM# 24505924 - Retail $18.20 from a dealership, may be more or less being in Canada.
For my car idle in a Texas/Ok summer in traffic on a 105*+ day the temp would see around 220* with the factory thermostat. This is why I dropped to a 180*. That being said, Winnipeg, CA would be much colder most of the year so you may be fine with just a factory 195* thermo, but I do NOT believe they make a compatible hotter one. She is just gonna run cool up there.
Replace the thermostat, flush the coolant and replace and see if she runs warmer sooner, you may just have to allow more time to warm up to operating temps.