Those are really cute! I found the silver one on ebay for $37.99 +$9 s/h. It is on a BIN so you don't have to bid.
http://cgi.ebay.com/NEW-PET-BICYCLE-...QQcmdZViewItem
Good luck! We bought the carrier that we pull behind the bike since we have 3! They LOVE IT!!!
bike trailor.jpg