Logitech sells some replacement receivers (usually $10 + S/H). Contact Logitech Sales at 800-231-7717 (US and Canada only), see the Contact page, or check the Support->Downloads + Parts section as some models now list receivers there for sale.
What model of mouse and receiver do you have left? Have you tried connecting them?
-Richard L. Owens, author of the uberOptions mod for SetPoint. Unlock the options of your SetPoint-controlled device! mirror