if you are asking about refilling CO2 tanks, you can try a fire extinguisher service center or a beverage center, that is if you want to own your own tank. if you don't mind a steel tank that you swap when you fill it you can try a welding supply company.
if you are talking about tanks here are two retailers
Green Leaf Aquariums aka GLA
i have done business with both all positive i have a 20# from the second supplier.