Where can I get a SIM card in Sarria or Santiago?

I think you are asking two questions:

1. SIM Card You can purchase a SIM card in any cell phone store. You will find them in Santiago, Sarria, and everywhere else along the way.
Check out Spanish SIM cards on google to get an idea of cost of the loaded card and the tarrif for making calls. That way you can make some decision as to which you might want before you get there.

2. Sleeping Bag in October/November: I think all would agree that the answer is yes. I would advise a silk sleep sack and a light weight (as in how much it weighs) sleeping bag.