Traditional Italian limoncello is typical of the Amalfi coast, a beautiful area around Naples. It's traditionally made with Sorrento lemons, which are large and flavorful and a really unique Italian product.
As said, ideally you would use Sorrento PGI (Protected Geographical Indication) lemons. If you can't find them, make sure to select large, ripe, flavorful lemons, and all the better if unwaxed and organic.
190 proof alcohol is sometimes available online or found in speciality shops, such as Polish or Eastern European shops. Note that this highly concentrated alcohol can be dangerous if consumed on its own, but here it is being diluted substantially and served in small portions.