There are many easily obtained inexpensive (and legal) radioactive sources available online.See video samples / discuss radioactive sources at http://forum.nothinglabs.com
Certain years of Fiestaware dishes featured a "radioactive red" color that actually contained uranium!
A small chip can make an excellent cheap cloud chamber source - try searching ebay for "fiestaware sample"
Small Autunite samples commonly sell for about $20 on ebay (contains uranium).
Search ebay descriptions for "geiger" and "cpm" to see other possibilities.
Alternately - the cloud chamber can be used without a radiation source to view cosmic rays. They typically appear once every 1-2 minutes.
Take precautions appropriate for your sample! The provider of your specimen may be able to provide further information on its safety.
To be extra safe - use gloves when handling. Wash your hands after if you should touch it.