You can do this easy.
This code is for bender theme but you can modify to match your theme.
Add this in your theme functions.php file
function search_by_currency(){
if (Params::getParam('currency') != '') {
$value = Params::getParam('currency');
Search::newInstance()->addConditions(sprintf("%st_item.fk_c_currency_code = '%s' ", DB_TABLE_PREFIX, $value));
}
}
osc_add_hook('search_conditions', 'search_by_currency');
And now the code for search-sidebar from your theme
<fieldset>
<?php
$currency_check = osc_get_currencies();
$number_of_currency = count($currency_check);
if ($number_of_currency > 1) {
?>
<h3><?php _e('Select currency', 'bender'); ?></h3>
<select name="currency" >
<option value=""><?php _e('All currency', 'bender'); ?></option>
<?php foreach ($currency_check as $curr) { ?>
<option value="<?php echo $curr['pk_c_code']; ?>" <?php if(Params::getParam('currency') == $curr['pk_c_code']){ echo 'selected';}?>><?php echo $curr['pk_c_code']; ?></option>
<?php } ?>
</select>
<?php } ?>
</fieldset>