Actually you will be lucky if you can find anyone who will be willing to fill it. Most welding suppliers only do exchanges, and are limited in the size tanks they carry. A typical welder uses a 20 pound or larger. Ten pound maybe, anything smaller than that unlikely. All the welding suppliers I ever contacted in California and Oregon refuse to fill your tank. According to them it is too time consuming and expensive to fire up their system just to fill one tank. When they fire up their system they fill dozens of tanks, and then swap these to their customers empty tanks.
Why would you want a tank so small? I dont quite agree with Niko. What is the most amount of C02 you might pump in? 2 bubbles a second? What is the least amount 1 bubble every two seconds? I don't think you would see that much of a time difference between the two.