The iSight camera IS replaceable but it's not a common party by any means since it wasn't available from Apple as a standalone part, it was only available as part of the display assembly.
A quick search turned one up on eBay - [http://www.ebay.com/itm/Genuine-NEW-Apple-Macbook-Pro-Unibody-13-A1278-iSight-Camera-Webcam-/270984764035|MacBook Pro A1278 iSight Camera]. I would obviously exercise caution when buying a part online.
This repair guide should walk you through the majority of the repair, I would recommend studying it before you order a part to ensure you're comfortable performing the repair. Basically you can see the camera module in step 25.
[guide|4311|MacBook Pro LCD replacement guide]