If you used to rely on Wal-Mart to get your CO2 tanks re-filled, you've most likely already discovered they've quit offering this service. In fact, the word is they're going to stop selling paintball equipment altogether. So you might become stuck without a place to re-fill your air tanks; don't worry! Consider this short list of possible places for re-fills...

Paintball Stores.  If you have a paintball shop in your city, these are the first places to look.  If they don't fill your tanks, they're sure to know who will.  You can also stock up on paintball supplies when you're there.  Find a local paintball supply store via the Internet or your phonebook.
Gun or Firearm Shops.  Many gun and firearm stores will fill up CO2 tanks.  Some gun stores sell air rifles and even paintball guns, so there's a chance they might also fill tanks.  While few cities have paintball fields or stores, most have gun shops.
Bass Pro Shops.  This is a giant store that may or may not be close enough to you to visit, but they sell and refill CO2 tanks.
Fire Extinguisher Supply.  Find out if there's a store or company in your area that refills/sells fire extinguishers.  Places that sell or repair fire extinguishers have the tools necessary for refilling a CO2 tank.
Fire Department.  Sometimes your local fire department can/will fill CO2 tanks.  If they do, it's doubtful they will have the right connections; you will want to purchase a CO2 refill kit before going.  This product includes all the necessary hoses and gauges to refill your tank.
Commercial Air Tank Supply Companies.  Find out where your local hospital gets their tanks refilled.  There is a good chance you will be able to get your tanks filled there as well.