Have you tried the journals like AWHONN(association of women's health, obstetric and neonatal nurses; ACOG(american college of ob/gyn's); ACNM (american college of nurse midwives), etc.?? Or contacting the associations themselves?
If you google it you come up with a bunch of stuff, they are mostly casual articles but some of them may list their references and give you a jumping off point.
This one, for example, lists "Source: The Maternity Center Association's Listening to Mothers survey" (a national survey):
http://www.parentsplace.com/pregnanc...ival_freqCap=2
Have you searched phrases like "percent women using drugs in labor" in your school's online database of full text journal articles? Using Cinahl, Ovid, ProQuest, etc.?