For the OP , some places will convert old fire extinguishers into reservoirs for the same purpose.I have had a Webley Axsor for 18 years:
View attachment 97849
It's a .22 with 8 shot magazine, CO2 powered and filled to 180 psi, using a diving cylinder, which lasts years - no manual pumping! The cylinder has to be tested every so often, mind.
It's a great gun and I would recommend it, but it not at all light. Very powerful and has taken out rats at 25m in the UK ... no rats around here, but too many bl**dy moles ... and, yes, I have tried the Jasper Carrot method with the shotgun, but without the revolving stool.
You get used to the weight of the gun, as in everything. For small vermin, it is very effective.