there's this site called Coolspotters -

where you can find the EXACT products the celebs are using and wearing both on and off the screen
full information about the certain dress,shoes,bag,etc.. there are also additional infos like where they shop,brands they use,cars they drive,places they visit and other celebs they work with -all of them are spotted by the fans

and Leighton has a profile with almost 1200+ spots

link

and for the Gossip Girl show with almost 1000 spots

link