Dusty,
No need to wait until your arrival in Astorga. In the United States you can have a Credential mailed to you in advance from the American Pilgrims on the Camino Fraternity. Go to
http://www.americanpilgrims.com. There is no charge although you can join the fraternity or any donation is welcomed.
By the way, I assume you mean Credential rather than Compostela. The Compostela is only issued in Santiago de Compostela upon presentation of your Credential bearing appropriate stamps from the Camino.
Buen Camino