Yup, you can pick one up at any Apple store for $19.
After reading several reviews online, I settled for a Hitachi Travelstar 7k500 (500GB @ 7200RPM). Works great, it's fast and makes no noise or vibrations, but it's a bit of a crap shoot with any HDD, you'll always find someone whose HDD failed, no matter which model. Check out some reviews and buy the one you like, shop at Amazon or newegg.com for good prices and peace of mind. Start with how much memory you want/need. You can pick up a 500GB 7200RPM HDD under $100 online.
Then you have two options: use the original HDD as a backup (external) HDD, which should last you a while (space wise). All you need here is buy an external HDD enclosure. Most USB 2.0 enclosures sell for under ~$25 (no need for firewire unless you work off your external HDD daily with large files). USB 2.0 is good for personal backups, update once or twice per week. This should be enough to get you started. Upgrade once your data becomes larger than the HDD capacity. By then you'll have more options and cheaper prices out on the market.
Or, you can just buy an external HDD (and keep the original as a spare HDD part, or sell it off to recoup some $$). Here you have two options as well: buy a "full product", i.e. external HDD fully assembled single product, or build your own (buy any HDD you like and any enclosure to house it).
Christmas come early for you this year