When sourcing stretch denim, the fiber blend ratio is the most critical factor. High-quality stretch jeans fabric typically contains 90-98% cotton for breathability and 2-10% elastane (Spandex/Lycra) for flexibility. For premium 'power stretch' variants, look for polyester blends (T400) which improve shape retention and prevent the fabric from bagging out at the knees. Always request a fabric weight specification, usually measured in ounces (oz); 9-12 oz is standard for year-round jeans, while >12 oz is considered heavy-duty.