This could have been written by me! I have the same question. I shop at those stores too and love the gossip girl style (of course to pull it of you have to have tons of confidence). Here's what I've found:
1. The gossip girl website has fashion video tips.
2. Try forever 21, alloy.com, wet seal, lulusfashionlounge.com, pinkicestore.com.
3. Macy's and Norstrom's have very cute Serena like clothing in their juniors departments and they have amazing sales all the time (every day).
4. For Serena's hair, curl you hair with hot irons.
5. Try to mix and match to make outfits out of the clothes you already have! It is easier than you think and it's super fun to just pile on the accessories and the colors.
6. Don't forget the jewelry! Serena usually wears long necklaces.
Try this
http://www1.macys.com/catalog/index.ognc?CategoryID=18109&PageID=18109*1*24*-1*-1&kw=Dresses&LinkType=EverGreen
http://www1.macys.com/catalog/product/index.ognc?ID=254841&CategoryID=24281&LinkType=EverGreen
http://www1.macys.com/catalog/product/index.ognc?ID=254839&CategoryID=24281&LinkType=EverGreen
http://www1.macys.com/catalog/product/index.ognc?ID=274519&CategoryID=24279
http://www1.macys.com/catalog/product/index.ognc?ID=274587&CategoryID=24279
http://www1.macys.com/catalog/product/index.ognc?ID=253324&CategoryID=24279
http://shop.nordstrom.com/C/6008190/0~2378467~2378483~2377032~6008190?mediumthumbnail=Y&viewAll=1&tuid=00000418-006c-085e-6361-745f6d656469&catID=6008190&Category=6008190
http://shop.nordstrom.com/S/2964773/0~2378467~2378483~2383026?mediumthumbnail=Y&origin=category&searchtype=&pbo=2383026&P=1
http://shop.nordstrom.com/S/2952416/0~2378467~2378483~2383026?mediumthumbnail=Y&origin=category&searchtype=&pbo=2383026&P=1
http://www.lulus.com/products/Zad+Black+Satin+Gemstone+Headband/6753.html
http://www.lulus.com/products/Adorable+Multi+Rhinestone+Headband+in+Gold+and+Silver/7237.html
http://www.lulus.com/products/Triple+Threat+Hand+Clutch/7355.html
http://www.lulus.com/categories/view-all/jewelry/100.html
http://www.lulus.com/categories/leggings/99_229.html
http://store.alloy.com/item.do?categoryID=11&itemID=48589&sizeFilter=&brandFilter=&incmpid=apparel.dresses
http://store.alloy.com/item.do?categoryID=11&itemID=48593&sizeFilter=&colorFilter=&brandFilter=
this dress above is just like the one serena wore in the episode poison ivy.
http://store.alloy.com/item.do?categoryID=22&itemID=48452&sizeFilter=&colorFilter=&brandFilter=
http://store.alloy.com/item.do?categoryID=1678&itemID=48515&sizeFilter=&colorFilter=&brandFilter=
http://store.alloy.com/browse.do?categoryID=1678&sidenavTrack=apparel.accessories.jewelry
Oh and lorrick is the real version of the waldorf brand!
alright well there's a bunch more, but that's all for now maybe i'll add some more later. Remember just play around with it and with the clothes you already have!
Alright well enjoy gossip girl, hopefully they will have some new episodes soon! <333