I looked up substitutes (Cook's Thesaurus) and learned that daun salam is also known as Indonesian bay leaf, and curry leaves or bay leaves are suggested substitutes, if you can't find duan salam. (But Spice Garden disagrees with the bay leaf suggestion.)
Spicetrekekrs said salam leaves are used just like bay leaves, and mentions that they are odorless and only add flavor with long cooking.
Spice Garden said they are often labeled "Indian Bay Leaf," so maybe if you look for that, instead of for duan salam, you might have more success. This site says curry leaf is a pretty good substitute, but do NOT use bay leaves, as they are too aromatic, and duan salam is extremely subtle.
And, of course, there are people selling them on line. Such as:
http://www.indomerchant.com/inbayleavdau.htmlHope that helps.