Most swimming pool shops in both granular and tablets.
I use a little shop in pano paphos. Drive down "gasoline alley " turn right at "Lukoil" . After 70 meters turn right into big car park and bus station. Drive to back of carpark and there is a general store with an electric sack ramp.
The lady is called Georgia, you won't see the salt you have to ask her. She speaks very little English but knows salt & tablets if you say it slow enough (thelo Allati tableta, if you want to go native). She is the cheapest in Paphos.
For very hard water one sack lasts about 6 weeks. My advice is don't fill the softener just top it with salt as needed half weekly or you waste salt. Possibly the newer models have sorted that problem?