Replacement plug on MagSafe adapter?
If you take the "broken" piece in to the store (or call in to AppleCare) they can sell you a "replacement" piece as a "repair" part. Don't worry - it is a brand new piece, not something that is repaired. I think they cost around $10.

I don't think you can order just that piece - but it is included in the updated Apple World Travel Adapter Kit along with an iPad charger, USB cord and the worldwide plugs. This kit is $39 in the US.