A workaround for ExpandableListViews to use the SectionIndexer to provide fast scroll ability. This assumes that you use the groups as sections.

ExpandableListView with SectionIndexer

Caveats

This implementation uses the OnScrollListener to watch the scrolling state. If you need to set your custom OnScrollListener to perform other logic you have either to replace the implementing interface with a concrete constructor argument or you have to put the logic into the adapter also, if possible.

As far as I know it is not possible to set another OnScrollListener without replacing the first one.

MyExpandableListAdapter.java

public class MyExpandableListAdapter extends extends BaseExpandableListAdapter 
                                     implements SectionIndexer, AbsListView.OnScrollListener {

    private final ExpandableListView expandableListView;
    private final Context context;
    private boolean manualScroll;
    private String[] groups;

    /* 
     *  Your other fields
     *      ...
     */
    
    public MyExpandableListAdapter(Context context, ExpandableListView expandableListView /* Your other arguments */) {
        this.context = context;
        this.expandableListView = expandableListView;
        this.expandableListView.setOnScrollListener(this);
    }

    @Override
    public void onScrollStateChanged(AbsListView view, int scrollState) {
        // toggle the boolean flag to indicate whether the user scrolled
        // manually via touch or used the fast scrollbars
        this.manualScroll = scrollState == SCROLL_STATE_TOUCH_SCROLL;
    }

    @Override
    public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {}

    @Override
    public String[] getSections() {
        // Provide your custom sections here
        return groups;
    }

    /* 
     * This method will get called either if the user scrolls manually or if he scrolls 
     * using the fast scrollbars.
     */
    @Override
    public int getPositionForSection(int section) {
        if (manualScroll) {
            // If we are scrolling manually return only the section
            return section;
        } else {
            // If we are scrolling via fast scrollbars get the packed position from the group(section) and
            // transform it into a flat position
            return expandableListView.getFlatListPosition(ExpandableListView.getPackedPositionForGroup(section));
        }
    }

    /*
     * This method will only be called if the user scrolls the list manually.
     * After this method #getPositionForSection() will be called
     */
    @Override
    public int getSectionForPosition(int position) {
        // Get the packed position of the provided flat one and find the corresponding goup
        return ExpandableListView.getPackedPositionGroup(expandableListView
                .getExpandableListPosition(position));
    }
    
    /* 
     *  Your adapter logic here
     *     ...
     */

}