Most places that sell paintball equipment can fill the cartridges. Additionally, look for any paintball fields in your area that rent gear, they typically can fill them as well.
Many sporting goods stores fill CO2 tanks. Some hobby stores in your area might,also. Other sources could include welding or industrial gas suppliers. Any local paintball field in your area will also be able to fill tanks,but some might not do so unless you are going to play on their field. (This is to discourage vandalism. )