Radioactive sources are available from a number of suppliers. We recommend Spectrum Techniques highly.

Spectrum Techniques
106 Union Valley Road
Oak Ridge, TN 37830
(865) 482-9937
Fax: (865) 483-0473

Note that their website does not list prices, but they will respond quickly to emails or phone calls with full information.

Also of note is an interesting website that describes radioactive sources in everyday objects: