Hi Roy,
I have two suggestions. There may be more options, if so I hope people will post them here.
1) Tour'n'Ride is a company located in new town. 5 minute walk from old town. They rent out bikes to pilgrims, ship the bike to your starting point then the pilgrims bikes it back to Santiago. They probably have boxes to sell you, or can give you some info on where to get them. I have talked to one of the guys that work here and I know he speaks english quite well.
Tour'n'Ride S.C.
Calle General Pardiñas, nº15 Bajo 8 (Galerias Bankinter)
Santiago de Compostela
Teléfono: 981.936.616 Fax: 981.937.539
Móvil: 687.887.521
info@tournride.com
http://www.tournride.com/english/index.php
2) Mailboxes ETC.
As you may know, from Mailboxes ETC. you can mail anything home (with UPS). They have boxes in all shapes and forms. I know they ship bikes, so you can probably buy your box here. I have also talked to the owners here, good people.
These guys are also located 5-10 minutes from old town, in new town. Walk from old town towards the train station, "Santiago de Guayaquil" is one of the streets on your left.
Mail Boxes Etc. A Coruña
Santiago de Guayaquil, 5
15702 Santiago de Compostela
Tel.: 981-938595
mbe127@adabel.net
http://www.mbe.es/franchise.php?c=SPMBE0127
I hope this helps,
Ivar