Greek bouzoukis are available from on-line instrument stores such as Hobgoblin, Thomann, Musikalia or even Amazon, but for the best quality instruments you need to get them from Greece. There are a few hand-makers of Bouzoukis and there are a few companies that mass produce them.
The Bouzouki Shop
Polydorou has two locations in Athens and goes under the name of 'The Bouzouki Shop'. Their main shop for selling the instruments is at 31 Pandrosou Street, Monastiraki (just to the east of the junction with Aiolou Street). This is the heart of the Athens Tourist Market, but the shop stocks professional instruments.
They also have a workshop in another part of Athens: 12 Logotheti Str. & Panagopoulou, Egaleo (5-minute walk from Egaleo Metro Station).
Aristotelous Street 5, Athens (Near Platia Vathis). West side of street, just south of junction with Kapodistriou.
Karolos Tsakirian comes from a family of bouzouki-makers. His grandfather Agop was an Armenian bouzouki maker in Smyrna (Izmir), Turkey. In 1922 he migrated to Piraeus, Greece, (along with a large number of other Christians who were expelled from Turkey). In Piraeus, he set up a new bouzouki-making business. Karolos's father Onnik moved the shop up the road to Athens in 1961. Onnik's bouzoukis were well-known, being played by such players as Manolis Chiotis. Karalos worked for many years in New York but returned to Athens in 1993 and took over the business, opening a bigger shop next door to the original. The shop is in a somewhat dilapidated part of Athens near Omonia Square. They make as well as sell instruments.
The website was down for much of 2018, but seems to be back working again as of Jan 2019.
This is one of the mass producers, possibly the only mass producer. Their bouzoukis also go by the name of Sakis (a diminutive of the company founder's first name). Based just outside the village of Mastro to the west of the city of Mesolonghi in Western Greece. These instruments are "hand-made" on machines in a factory. There are many different models - their website lists 23 different types of 6-string bouzouki alone, ranging from about €250 to €2,500.
Matsikas has a large shop in Athens: Kremou 40, Kallithea, which is between the Acropolis and Piraeus. Very close to the Taúros metro station (line M1).