Lucky you, great gift.
Before I laid down money for a hard drive, I would also wonder if notebook might have some other issue, like has it been dropped? Does it show any physical damage? Has it gotten liquids into it? It might need a new battery, I sure would not worry about replacing a battery until I verified it worked.
I hope someone who has experience with this MBP can come along and suggest how to test the computer before you buy a drive, or perhaps the person who gave you the computer can verify to you that the MBP was working correctly recently.
Anyway, in response to your questions. You can buy a hard drive from places like Apple itself,
Welcome to the Apple Store - Apple Store (U.S.)
For a bit less money you could buy one from OWC
Performance Upgrades; FireWire USB SATA Storage; Memory, more at OWC No I am not an employee, just a satisfied customer, and many here have with purchased from OWC with success. Both OWC and Apple Store will help you to be sure you get the right drive. OWC has links to videos and written instructions about how to install the hard drive. Also
iFixit: The free repair manual
One can also find a deal on a hard drive from newegg,
Newegg.com - Computer Parts, Laptops, Electronics, HDTVs, Digital Cameras and More!
Newegg will not help you choose the right hard drive, and beware, some of their offerings are not correctly listed. Newegg is for geeks who know what they want, and it is considered to be a very trustworthy company who will quickly take back defective parts. For you, I think the interesting thing about newegg is the section showing feedback on whether a particular part is a good purchase, has problems and such. If you find the hard drive on OWC, go over to Newegg, enter the info and look at the feedback for that hard drive.
If this computer has not been used in awhile, then you might have some unusual issues while trying to install Snow Leopard. So do not get discouraged as they can usually be solved simply, but maybe not with the "how to install guides" meant for a working computer. Uh, I guess you do not have a best friend who is an Geek who has an Apple?
How to format the hard drive with the Snow Leopard disc.
Performance Upgrades; FireWire USB SATA Storage; Memory, more at OWC
Else, written instructions on how to install Snow:
http://manuals.info.apple.com/en_US/Snow_Leopard_Installation_Instructions.pdf
Apple - Support has links to info.
There are other how to's that you can google for.
I see I missed saying it, Apple is a trustworthy supplier, and Apple is the Gold Standard in quality in product, people. (Apple Store does charge more for some things.)
Have fun.
