Number Slider for exposed filter using Drupal 6
Better Exposed Filters has that functionality built in


  ...
  
  What else can I do with BEF?
  
  
  Use the jQuery Slider widget for greater-than, less-than or in-between types of filters
  


I've never used it for D6 but the project page doesn't mention the slider being missing from that version or anything so it should be there.