The Almagest tells us that Ptolemy had access to the works of Hipparchus, Aristarchus of Samos, Timocharis, and Aristillus who probably all worked for the Musaeum of Alexandria at one time and its companion institution the Library of Alexandria. One of the functions of the Musaeum was that of an astronomical institute. Ptolemy had access to, studied and added to these records and the result was the Almagest. But precisely where did Ptolemy work?
The Library of Alexandria was distroyed 48 BC in when the Romans invaded Egypt. It is claimed that the neighboring Musaeum survived. When Mark Anthony later ruled in Egypt he presented the Library of Pergamon to Cleopatra as a gift. The remnants of the Library were moved to the vicinity of the Serapeum, a temple dedicated to the Greek/Egyptian god Serapis.
The century after Ptolemy's time we know that the Brucheion, the Royal Quarter in Alexandria, was distroyed by Aurelian. It is debatable whether or not the Musaeum still existed at this time. It is possible that some of the scholars from Alexandria, such as Sosigenes, went to Rome and assisted Caesar with his calendar reforms. But Ptolemy's work is primarily associated with Alexandria and it may have retained some of its past glory. The most likely sites associated with Ptolemy are those of the Musaeum or the Serapeum.
A note on Ptolemy's name. In his Geography there are two places named Ptolemais listed, one in Arsinoites Nome, a naval station, at 61° 40', 29° 20' (Ptolemy's coordinates) just south of Arsinoe at 61° 40', 29° 30' and another Ptolemais Hermiae in Thinites Nome and its major city at 61° 50', 27° 10'. Ptolemy's coordinates list the latitude after the longitude which is measured eastward from the Canary Islands. His longitudes appear to be a little too large. It is believed that Ptolemy was from Upper Egypt.