I'd recommend against buying off-branded ones on Ebay, because they have cloned IDs. Microsoft hasn't, to the best of my knowledge, locked these out or banned anyone for using them, but they conceivably could.
On the other hand, you can pretty easily crack open an old-style 360 HDD and put it in a slim, and the system can't tell the difference at all. They're all Microsoft hard drives, they're just in different cases. So, I'd go to Gamestop and get a used 120GB for $40 and pop it in.