Hi Ann
I would you suggest you try the CAMPER PARK Santiago La Ribera at San Blas , they sell gas there are very helpful people and quite possibly will have what you want, or at least give you an answer as to whether the smaller bottles are available here.
If they do have them I assume it would not be an exchange as uk bottles have a different fitting to here.
This is their web site http://camperparklaribera.com/en/home/ maybe you can phone them first, but personaly I would pay a visit.
Hope this helps