I've had success with the following Progress brand Luminaires:
P5818-30 WHITE POLYPROYLENE WALL LANTERN $8.00
P5818-31 BLACK POLYPROYLENE WALL LANTERN $8.00
Which mate up beautifully with the following WP box:
Thomas and Betts TR114WH Box Round Weather Proof White W/5-1/2 KO $4.69
I haven't confirmed any other good fits, but it seems to be a standardized thing somewhat...