eBay can be fine, just make sure you get a physical box, because Apple considers the card in that box the ultimate proof of ownership. I didn't with my most recent purchase, and it applied fine, but I haven't actually tried to claim on the warranty yet, so I'm crossing my fingers. There was a whole thread on this that's still active.
If you want to be a little more certain and are willing to spend a bit more to gain that certainty, try Amazon. It'll be a bit more (probably $50-75 more), but it'll be brand-new unopened box so there's no risk. Still cheaper than Apple by a decent amount.
EDIT: eBay has them for around $160, Amazon was $258, so $100 difference. Still, for the no-risk option, Amazon might be worth the extra money.
jW