You must have an AVSIM account in order to get and they are free.
The file is near the bottom of the page. If the link does not work, go to AVSIM and search the name "Jeff Favignano" Once the results show up, scroll down and you will see the file titled Longer contrails. Install instructions are in the readme that comes with the file. Just replace the existing two contrail files already in the effects folder.