No stamp, get one at the Casa de Cultura on the far end of town (open late morning and has WiFi).
I arrived a Sunday with the albergue closed. There is a very nice hostal further down the main street (c. real 62), Quiper Hostal, just above the bar. Ring the door bell. I paid €17 for a nice room with private bathroom. They have stamp/sellos as well.
A very large albergue that cost 10 euros. Large group of children here but I was put in a separate dormitory as far away as possible. Water is rust coloured and barely warm. Otherwise it is fine.