Add a tax_query to your $args
…
$args = array(
'tax_query' => array(
array(
'taxonomy' => 'product_cat',
'field' => 'slug', //possible values are term_id, name, slug or term_taxonomy_id
'terms' => 'tshirt' //can be single string or array of slugs, names, term_ids or taxonomy_ids
)
)
);
$results = get_posts( $args );
Recommended reading: