Where can I get my CO2 Tank Filled - This link takes you to a listing of National Welders supplies and gases. After entering your zip code, make sure you call before going over.
Also, per the posts below. Fire Extenguisher Shops and Soda and Beer distributors are another source. In fact, My last fill was from a Fire Extenguisher sjop, but I have used some of the welding shops listed in the link as well.
I added this whole thread to the 80's section FAQ in the General section.