Hi, pilgr,
I would have suggested that you ask Vanesa at Pensión Paris to get you one, but it's too late for that if you're on your way! She has done that for a number of pilgrims. But now let's think about Plan B.
Try contacting the Valencia Amigos --
Dr. Gil y Morte, 24-1º-3ª
46007 Valencia
Tel.: 96 385 99 82
Fax: 96 313 01 79
info@vieiragrino.com
I think they are only open on Friday evenings though.
Here is a thread that says no credenciales are available in the cathedral. That was my impression as well.
https://www.caminodesantiago.me/community/threads/recent-camino.26572/
Good luck! If you don't find one in Valencia, just keep a piece of paper with stamps and hopefully the albergue in Algemesí will have one. Buen camino, Laurie