These come directly from the manufactourers (sp?) They charge exuberant prices for them but the dealerships probably don't care about that so much. They don't make a whole lot of money off of them since they are already priced so high.
These stripes are available on the 2009 and 2010 Challanger, but not on the 2011.
The part numbers will vary depending on the original paint code, but the are either:
1LW72GS4AA (right) and 1LW73GS4AA (left) (silver)
1LW72RXFAA (right) and 1LW73RXFAA (left) (Black)
I've reviewed both the service documents and the parts catalog and neither have the information you are looking for (I have access to the same factory information dealerships do). Short of buying a set of decals, your best bet would be to walk the dealers lot with a tape measure and a note pad.