Is there an alternative to condensed milk?
To make sweetened condensed milk:


  The best make-your-own version is to
  mix 1 cup of evaporated milk with
  1-1/4 cups of sugar in a saucepan,
  heat and stir until the sugar is
  completely dissolved, and let cool.
  
  If you don't have evaporated milk on
  hand either, you can make your own by
  slowly simmering any quantity of milk
  in a pan until it reduced by 60%, and
  then adding the sugar.


Source: http://www.ochef.com/125.htm

Another recipe that sounds like a lot more work and uses powdered milk: http://www.ehow.com/how_4903555_make-condensed-milk.html