var store = new Ext.data.Store({
     model: 'Result',
     proxy: {
         type: 'ajax',
         extraParams: {search_term : term},
         url : 'find.pl'
     },
});

store.load({
    params:{
        search_term:'my runtime search term'
    }
    //other options like a callback function, append/add flag, etc.
});