Linked options helper methods for Shopify. See this: http://wiki.shopify.com/Linked_Options

gistfile1.html

<script>
// (c) Copyright 2011 Caroline Schnapp. All Rights Reserved. Contact: [email protected]
// See http://wiki.shopify.com/Linked_Options
Shopify.optionsMap = {};
Shopify.updateOptionsInSelector = function(selectorIndex) {
  var key = jQuery('.single-option-selector:eq(0)').val();
  if (selectorIndex === 2) {
    key += ' / ' + jQuery('.single-option-selector:eq(1)').val();
  }
  var selector = jQuery('.single-option-selector:eq(' + selectorIndex + ')');
  var initialValue = selector.val();
  selector.empty();    
  var availableOptions = Shopify.optionsMap[key];
  for (var i=0; i<availableOptions.length; i++) {
    var option = availableOptions[i];
    var newOption = jQuery("<option></option>").val(option).html(option);
    selector.append(newOption);
  }
  if (jQuery.inArray(initialValue, availableOptions) !== -1) {
    selector.val(initialValue);
  }
  selector.trigger('change');
};
Shopify.linkOptionSelectors = function(product) {
  // If we have only one option, we have nothing to do.
  if (product.options.length === 1) return;
  // Building our mapping object.
  for (var i=0; i<product.variants.length; i++) {
    var variant = product.variants[i];
    if (variant.available) {
      var key = variant.option1;
      Shopify.optionsMap[key] = Shopify.optionsMap[key] || [];
      Shopify.optionsMap[key].push(variant.option2);
      Shopify.optionsMap[key] = Shopify.uniq(Shopify.optionsMap[key]);
      if (product.options.length === 3) {
        var key = variant.option1 + ' / ' + variant.option2;
        Shopify.optionsMap[key] = Shopify.optionsMap[key] || [];
        Shopify.optionsMap[key].push(variant.option3);
        Shopify.optionsMap[key] = Shopify.uniq(Shopify.optionsMap[key]);
      }
    }
  }
  // Update options right away.
  Shopify.updateOptionsInSelector(1);
  if (product.options.length === 3) Shopify.updateOptionsInSelector(2);
  // When there is an update in the first dropdown.
  jQuery(".single-option-selector:eq(0)").change(function() {
    Shopify.updateOptionsInSelector(1);
    if (product.options.length === 3) Shopify.updateOptionsInSelector(2);
    return true;
  });
  // When there is an update in the second dropdown.
  jQuery(".single-option-selector:eq(1)").change(function() {
    if (product.options.length === 3) Shopify.updateOptionsInSelector(2);
    return true;
  });
};
</script>