iPhone 4 Back Glass Replacement

I dropped my iPhone recently and the back glass is now broken in the top right-hand corner.

I want to get it replaced but I live very far from any Apple Store, and don't want to have to send it in for repair. My sister lives near one and is coming to visit soon. Does anyone know if an Apple Store will allow her to purchase the back glass replacement for me and bring it back with her, or do they require physically having/seeing it?

Can anyone suggest a company that provides such a decent replacement part if this isn't an option?

Thanks!