Just look on Newegg for a 2.5" drive. Those use laptop drives, so they don't require extra power, only USB.
You'll be able to get something larger than 100gb, even 160GB are around $45. If you didn't mind the portability factor, you could use a 3.5" external hdd and get it for cheaper with more room. Right now, the largest you'll find in 2.5" will be 500GB maximum, though I'm guessing they're only up to 320 in the decent price ranges.
Here's an
example search. If you get it from NewEgg, you'll also get some sort of warranty unlike something from Ebay.