There are a four options that you can pursue for a suitable air pump for the LI-820 CO2 Analyzer. The main point to keep in mind with an Air Pump for this analyzer, is that the flow rate should not be more than 1 liter per minute (lpm). If the flow rate is higher than this, the instrument has a hard time keeping the cell/bench temperature stable and as a result, the CO2 Concentrations may vary and may not be completely accurate.
At LI-COR, we sell two different Air Pumps. These can be found inside the "Air Flow Parts List" (pdf) attachment.
(1) First, the Rotary Vane Adjustable Pump (part number: 286-07218). This Pump has a flow rate of 3.5 lpm when a DC power source is supplying 12 VDC. However, if you can control, or vary, the DC power source, then you can supply the Rotary Vane Pump with a DC voltage of 5 or 6 volts. At a lower input voltage, the pump's flow rate should drop to have a flow rate of 1.0 lpm or less.
(2) Second, we also sell a Diaphragm Pump (part number: 286-04198), which is a little less expensive. This pump's flow rate cannot be controlled by changing the voltage. However, you can use some plumbing techniques to ensure that only 1.0 lpm enters the LI-820. You will want to have a Rotameter to restrict the flow. Place this between the Air Pump and the LI-820 Analyzer. Then, just prior to the Rotameter, you will want to place a T-fitting. This third port of this 'T' should have a tube which vents out to the atmosphere. I would suggest a 6-inch long tube to prevent back-flow of air. The Air Pump will then push 3.5 lpm of air towards the LI-820 analyzer. However, the Rotameter will only allow 1.0 lpm to pass through into the analyzer. The remaining 2.5 lpm of air will vent out the tube prior to the Rotameter. I have also included a (jpg) drawing of this set up to help visualize how this would work.
(3) Third, there is another pump manufacturer listed in the (pdf) attachment. This company "Rena" also makes small pumps. However, this pump also does not have a low enough flow rate to use alone with the LI-820. It is powered by AC power, so you cannot manipulate the voltage levels to lower the flow. However, you can use the Rotameter and Vent method as described in the second option above. The benefits of this pump might be price and a lower starting flow rate (1.8 lpm).
(4) You can do some searching for an Air Pump on your own. Some users of our have found pumps at local pet shops. The aquarium pumps often have the desired flow rate of 1.0 lpm or less. You will want to make sure though, that they do not have leaks around/inside the pump. You could test this by blowing your high CO2 breath around the pump while it is running and see if the CO2 concentration in the LI-820 increases as a result. If so, then there are leaks.