Well, first of all, you're DEFINITELY going to want a 2.5" external drive, because it's way smaller than the 3.5" external drives AND it is powered by USB... so exactly what you said you wanted.
Some options for you...
1TB WD for $99.99
http://www.ncix.com/products/index.p...l&promoid=1270
640GB WD for $79.99
http://www.ncix.com/products/index.p...l&promoid=1270
However, the ones above are a little thicker than your standard 2.5" drives.
If you want to pay a little more, I think this one is thinner...
1TB WD for $117.99
http://www.ncix.com/products/index.p...l&promoid=1270