not many current new derailleurs, but there are plenty of older gear out there on ebay that will work just fine. Your shifters are simple friction levers so just about any front derailleur with the proper size clamp (28.6) should work just fine. Lots of older Shimano, Suntour and other brands on ebay that will replace your old simplex.

Here's a decent Shimano part: http://cgi.ebay.com/Shimano-Z202-Fro...QQcmdZViewItem