Although sometimes the flowers are called "tiger lily buds" they are in fact the the bud of the cultivated Day Lily. The flower has been used in Chinese cuisine for hundreds of years.
The buds are harvested when they are about 3 to 5 inches long and dried. Once dried the bud color changes to a golden yellow.
Lily buds (lily flowers) are typically available in cello bags the buds should be gold to medium (not dark) brown.
Lily buds should be stored in an airtight container, refrigerated. The buds will keep indefinitely.
Look for this product in a well-stocked Asian grocery store, otherwise you can buy them online at Amazon.com