You can do it yourself, it isn't hard at all to do, especially if you've recovery discs.
You can order a new disk from here:http://www.newegg.com/Store/SubCategory.aspx?SubCategory=380&name=Laptop-Hard-Drives
You do need to determine what capacity disk you want, how fast you want the disk to spin, and what interface it needs to use. Capacity wise, you can pretty much choose anything you want, but 320GB seems to be the spot more and more notebooks are filling. As for spin speed, you can choose 4200RPM to save power (but compromise speed), 7200RPM for best performance (but lower battery life), or 5400RPM to get the middle mix of the two. Usually that's what I go for. You need to determine, though, if the disk you have uses an ATA interface or SATA interface, and get a new disk with the same interface. You can't change that.