If it was me, I'd look on pricespy and find the cheapest retailer that also has a bricks and mortar store. There are a lot of fairly amateur retailers with online-only ordering that don't even accept credit cards, so I usually just go into a nearby store that has the item fairly cheap.
Go for the biggest one you can "afford". A 320GB 2.5" external HDD should only be about $160-190 so get one of those.