Hi, Komatic,
Welcome to the forum, and sorry about your pack. I hope Iberia is right.
I would hazard the guess that if you are going to find a credencial anywhere in Valladolid, it will be at the cathedral. Or at least they should able to tell you where to get it. But maybe someone else knows for sure. Since Valladolid isn't actually on any camino, it may be a bit more complicated, but surely there are lots of people from Vallodolid who walk and need a credencial.
I am going to walk the Camino de Madrid starting in a few weeks, so I have been looking at maps and information. I think that the best way to start the Camino de Madrid from Valladolid would be to take a bus to Simancas, which is only a few kms away, regular and frequent buses, and right on the route. You can head north from there. If you have access to a printer, there's a very good guide on the Confraternity of St. James' website. Here's the link:
http://www.csj.org.uk/guides-online.htm
It seems that the Camino de Madrid is very well marked, and that the albergue situation is very good north of Simancas.
By the time you get to Leon you should be in great hiking shape, so think about whether you want to walk from Leon to Oviedo on the Camino del Salvador. I don't know if that's what you were planning or not, but let me just cast one vote strongly in favor. It's spectacular, goes right up over the mountains from Castilla y Leon into Asturias. There is also a short online guide to that Camino on the same website.
Good luck with this, buen camino, Laurie