Just curious why you want a portable HDD for time machine? It's not really recommend to keep your backup with you and your computer, because if you lose the computer you're losing the backup too. Plus portable drives are more expensive, come in smaller capacities, and can be more prone to failure since they typically move around more.
Why not just get a WD mybook or similar? Cheap, reliable, effective.
I have a MBA 11 inch with 64GB. I picked up a 64GB jump drive that I use. I travel a lot and am concerned with crashes more than loosing my MAC. All of my data is backed up on the iCloud and Pogo. For me, carrying a 750 GB HDD to back up a 64GB or 128 GB, you get the idea.
I have a Verbatim external hard drive that I purchased two years ago at an Apple Store that I think is really portable. It's only about 256 GB and was a little less than $100, but that was two years ago, so you could find a larger capacity for much cheaper now. I've used it to back up my MacBook Pro and now my Air, and it has an input for both Firewire and USB. It also comes with an extra charger in case your computer doesn't supply enough power through the USB port to power the drive. My 11" Air though was able to power the drive through USB only, so I didn't need to use the charger.