Literally ANY place that transacts business with the public must have a rubber stamp identifying their business name and address. This is to generate manual receipts if the automated payment system fails.
If you cannot obtain a Camino-styled stamp / sello as described above, just go into any business, and ask to borrow their business rubber stamp (el sello para el negocio). Apply the stamp and then date it as you would another stamp obtained from a albergue, hostal, cafe, etc.
I have done this at a couple of gas stations over the years. It works.
But, I have found that if you get in the regular habit of obtaining a stamp at at least one place you stop for a break during the walking day, and again when you check into your lodings in the afternoon, you should be set - even for the last 100 km where you are required to have 2 stamps per day.
In fact, if you are walking a long distance, your credencial likely does not have enough space for two stamps per day for more than a month. So, either carry another credencial, blank pages fastened into your credencial , or focus on one daily stamp until you hit the 100 km mark (Sarria, Tui, Ferrol, Ourense, etc.). Then focus on getting two per day.
Hope this helps.
Tom