Use 4 diodes ( -| ) as shown below to make your own rectifier for older bikes! 
The "-" is the negative side of the Diode. These are available at Radio Shack 
or any electronics supply house. Be sure the Diode can handle the voltage and 
amperage needed for the job! 

Wire as indicated in the "Cheesy ASCII Drawing" (TM) below...

   Negitive
      /\
    -|  |-
AC <     > AC
    |-  -|
      \/
   Positive