Where are the cabin air filters on a 1999 Chevy Suburban 2500 with dual air?

The 1999 Chevy Suburban 2500 with dual air conditioning doesn't have a cabin air filter as a standard factory component. These were not commonly included in vehicles of that era unless it was a specific option or an aftermarket addition. To have cleaner air, you likely need to replace or clean the HVAC system's evaporator core and associated components.