You can purchase processors for laptops on sites such as SnapDeal.com and SparePartsWarehouse.com, as of October 2015. However, if you are buying a processor for replacement purposes, you must first determine the exact type of processor you have. If you are buying for upgrading purposes, you must determine what processors are compatible with your laptop's motherboard.
The best way of knowing what specifications your motherboard requires is finding out the specifications of your current processor. You also need to check your current processor's speed, its L2 cache and its bus speed. To find out your current processor's specifications, use programs such as CPU-Z. CPU-Z is stand-alone and doesn't require installation. Upon running, it shows the specifications and the speed of your current processor.
It's also important you consider the socket type of your current CPU. This has to do with the number of pins that your CPU has. If you purchase an incompatible processor, it may not fit in your laptop's CPU socket. Also, check whether your current processor is soldered on your motherboard. If this is the case, it is non-replaceable.
Be sure to consult your laptop's service manual before opening it. Be aware that upgrading or replacing your laptop's processor voids warranty.