RAM for most desktops now is 240pin, laptops are only 200pin. In lame man's terms, RAM for desktops is longer and won't fit into laptops. newegg.com has a good selection of RAM for laptops and their prices are usually good. Here's a link:
http://www.newegg.com/Store/SubCategory.aspx?SubCategory=381&name=Laptop-Memory
If you want help finding the specific kind you need, post the specs of the RAM and I can help you narrow your selections. If you don't know what RAM you have or need, download CPU-Z from CPUID.com. It will tell you what you need to know.