Hello. Seeing how it's still illegal in Italy
You'll need to hang outside the nightclubs or popular bars and approach people who are smoking, Carefully.
People often will slip out for a toke and then go back for a few more beers. Just be careful and ask nicely.
Try taking a few short taxi rides and asking some drivers, that you think smoke or might be cool and know where to buy some.
Not all will know. But some will. Just be careful.
Go to the popular parks through the day and follow any good cannabis smell... In a big city like Olbia you'll find someone smoking.
You'll need to do a little work, but you should be able to find it.
Good Luck.