In any city, stop at the Tourist Information Office (TI) and ask them. They may have them, or may direct you better. Another good source is the religious articles shop usually found in large churches or Cathedrals in major cities, like Segovia...
In the meanwhile, the blank paper approach works. Make sure you keep the sellos / stamps in the same order you would if the paper were glued into your credencial as an extension page. Many long distance pilgrims use this method. Their credentials frequently look like origami sculpture by the time they finish at Santiago.
Also, if you have a smartphone and are taking photos daily, the phone automatically dates, time and location stamps each photo. So, at the end , if you have to fill in some holes. the photo data may help. If this is an issue, point it out to the Pilgrim Office staff person and offer up your phone and the photos as evidence that you were at a particular place at a particular date and time.
Finally, for the purpose of qualifying for a Compostela, ONLY the final 100 km into Santiago on any accepted route is actually looked at to accept your claim. The additional distance you can document is used for a Certificate of Distance, if you want one... This is an optional document.
Relax and enjoy your Camino. It will all sort out...
Hope this helps.