Places that fill CO2 tanks include paintball stores, Bass Pro Shops stores, Dick's Sporting Goods stores and commercial air tank supply companies. Other places that may fill CO2 tanks include fire departments and gun shops.
Paintball stores refill CO2 tanks because CO2 is required to play paintball. If the paintball shop is not able to fill CO2 tanks, then store managers can probably provide recommendations for places that can.
Bass Pro Shops is a large hunting, fishing and outdoor camping retailer; Bass Pro Shops stores sell and refill CO2 tanks as well. Dick's Sporting Goods offer 20-ounce CO2 tank refills for approximately $5.