We carry CO2 tanks. Here's the 5 lb CO2 tank on our site.
If you decide not to go for new and want to look for a used one, allow me to suggest proceeding with caution. CO2 tanks (actually any high pressure cylinder) are required to be hydrostatically tested every five yrs. In addition, they are required to be visually inspected upon every fill up for rust, dents and seam aberrations.