The main issue will be that you may not have space for two hard drives to fit into the system. If this is the case and all you want is more storage space you have two options: buy an external hard drive, or buy a larger hard drive than the one which came with it (500gb) and replace it. Something like this: http://www.newegg.com/Product/Product.aspx?Item=N82E16822136545&cm_re=1tb_2.5-_-22-136-545-_-Product
As for where you can buy it from, that depends on where you are. Are you in the US? If so the link above (newegg) is a pretty good place to purchase from.