Have you tried realoem.com
Here's the full BMW Performance Aerodynamics kit
for the E87
and here's just the E87 BMW Performance Skirts
E81 BMWP Skirts
E82 BMWP Skirts
E88 BMWP Skirts
They have different part numbers, so I'd say that they aren't the same...
It looks like there is one type of skirts for the 2 door models and another for the E87...
Hope that helps