hello: how i got my one:
Merkato: From the Piazza area coming, thre is a about 5 level high shopping building on the right side. somethng with k, but you can not miss it. the only big builing. On the first flour they have guitars. and capo and strings.
I bought my one not there but cheaper on the way to the main mercato (coming out of this builing turn right. On the bay after about 100 meters there are some small shops with guitars.
i could imagine, that these shops are open on sunday. thay have western-style, not spanish.
good luck. Matthias