I'd suggest sicking to real Swing, nonswung songs are Ok for WCS but I wouldn't dance ECS to them. For slowish ECS, my picks would be:
The Pink Elephant by the Cherry Poppin' Daddies
The Cherry Poppin' Daddy Strut
Tuxedo Junction by Glenn Miller (the version I have is at the right tempo, at least)
Big Hair Mama by Indigo Swing
A little faster:
Red Light by Indigo Swing
The Best You Can by Indigo Swing
Begin the Beguine by Glenn Miller
Also there's LOTS of great songs in the right tempo range on Lily Wilde's CD, Insect Ball. Depending on where you are, the internet may be the only place to find it, though. I don't know about getting the Indigo Swing CDs (red light and all aboard).