This started happening to me in the past few months. Occasionally it would take two turns to start. Lately it's been a little more often and three times a charm. I have an 06 z4 and I ordered one from carparts.com for about $30 shipped. Here's a link.
http://www.carparts.com/details/BMW/...FdBahgodR3QHZA
I'll report back after I replace it to let you know if that's the issue.
-Jonathan