Craigslist. Although they commonly are listed for $1000-$1500, I've recently (~year) seen two for $500 including the rack to hold it when it's off the car. It's best to drive TO THE SELLER to pick it up, since safe shipping will likely cost as much as the top itself.
Quotes to live by:
guessing gets expensive
nothing is more expensive than a cheap BMW
Ken Kanne, Silverhill, AL, Honorary Forum Grandpa/Craigslist addict/Hoarder of all sorts of stuff
1995 318is "Bebe"; 1993 325is "Elvira" 1985 635CSi "Katja" 1984 633CSi "Sylvia"
I NEED A NAP, DANG IT!