Why not buy them new? It's never a good idea to run used pads or rotors.
http://cgi.ebay.com/ebaymotors/Bremb...Q5fAccessories Here's some cheap ones:
http://cgi.ebay.com/ebaymotors/BMW-R...Q5fAccessories Bimmerworld has front rotors for our cars on clearance for 20 bucks a pair.
The two links I posted were on ebay, but I wouldn't recommend just any ebay junk. I'd only run Brembo, Zimmermann or Genuine BMW. Also, IIRC Z3's use the same rotors as the ti, so you might be able to expand your search a little more.