I have a Geiger counter from the 1960's that includes a small sample of radioactive material on its side for testing and calibration. The manual states that there should be a certain number of clicks per second, and based on the half life of the material, it looks like it still works fine.
Amazon also sells small samples of uranium that have a specific number of clicks per second that you can use to test your equipment.
Other than that, there is a normal level of background radiation that amounts to about 14 clicks per minute if no other material is available, but this might not be viable in your area.
Actually, I don't really have a pressing need for my Geiger counter, and it sure sounds like you need it more than me. If you want, I'd be happy to ship it. Let me know.