Hi,
Are you referring to the front lcd glass/ bezel cover: http://www.ebay.com.sg/itm/New-Apple-MACBOOK-PRO-13-13-3-Front-LCD-Glass-Bezel-cover-A1278-A1342-/251162038036?pt=US_Laptop_Screens_LCD_Panels&hash=item3a7a6c8f14
This one listed on ebay cost around $28USD (brand new). Replacing it aint easy as it is held on by glue which you'll need a heat gun to soften the glue. Plus a lot of strength! >.<
If it is the hinge clutch cover: http://www.ebay.com.sg/itm/Macbook-Unibody-13-Non-Pro-A1278-Genuine-Hinge-Clutch-Cover-Assembly-GLP-/400385819978?pt=LH_DefaultDomain_0&hash=item5d38dac54a
This one is listed on ebay for about $16USD.
Replacing this is very easy.