Check the interweb to see exactly what drive you need. It will save untold grief.
Presumably it's a 2.5" drive, but there are a number of specifications such as SATA and PATA and good old ATA (God only knows what they mean).
There are also different speeds, the main being 4,200, 5,400 and 7,200. Faster drives work faster, but there may be a penalty in battery life. To complicate it further, a large slower drive can operate as fast as a smaller capacity higher speed drive, so possibly the best advice is to buy a large 5,400 rpm drive if possible.
As far as brands go I've found Seagate to be very satisfactory.
I've bought a variety of stuff from a few times and have found them fine, although
http://www.scan.co.uk might be cheaper with the exchange rate now.