Be patient with Barrett; he's not worth dropping for anything you're likely to find as a FA.
However, if you're just looking to add a temporary catcher to your bench, guys like Gerald Laird, Ronny Paulino, Chris Iannetta, Mike Napoli, Miguel Montero, and Carlos Ruiz are some of the most popular pure "sleeper" catchers. Other solid guys like Estrada and Molina could also be on your wire.
"When you don't feel good and you still get hits, that's when you know you are a bad man." -Manuel A. Ramirez