If we exclude the Rockies as sellers, six potential left-handed relief trade candidates jump out: Mark Hendrickson (Orioles), Pedro Feliciano (Mets), Doug Slaten (Nationals), Brian Tallet (Blue Jays), Brian Fuentes (Angels), and Mike Gonzalez (Orioles). Gonzalez has been injured much of the season so we won't look at his numbers. The samples are otherwise tiny for the other five southpaws, but all have pitched well against lefties in 2010 (xFIPs below 4.00).
Tallet, Fuentes, and Gonzalez are well-paid and likely to clear waivers. That might make trading them easier, if their clubs are willing to assume significant salary. Feliciano can probably be ruled out for two reasons: the Mets will be reluctant to help the Phillies, and as a Type B free agent he brings the possibility of draft pick compensation after the season. Hendrickson and Slaten are the best fits - they're cheap and controllable for next year. However, Hendrickson could be claimed by an American League team and Slaten would have to make it past the Dodgers, among others.