I brewed an All-Amarillo Pale Ale that was nearly an IPA about a year ago, and it turned out delicious. Many sources indicate that Amarillo is a suitable substitute for Centennial. While there may be closer substitutes, they should be reasonably similar to each other to produce a good product within the ballpark of the Two Hearted Ale - the end result will probably be a bit smoother and less heavy on the grapefruit character. So I'd brew it with what you've got.
If you want to go for maximum authenticity, I checked some other online stores, and many of them have Centennial available for reasonable prices - Nikobrew and Hops Direct, for instance. Amarillo is a great hop, and you'll always find another use for it in an American Pale Ale or West Coast IPA if you were to go ahead and use Centennial in your clone batch.