The famous printer Robertus Stephanus (Robert Estienne) published four editions of the Greek New Testament. All of these can be found online, though quality and access differs. The list below is not exhaustive, and will be updated as more information comes to my attention.
– Österreichische Nationalbibliothek, 1.L.29: ÖNB | GB
– Regensburg, Staatliche Bibliothek, 999/Script.7: MDZ
– Österreichische Nationalbibliothek, 1.L.34: ÖNB
– Universiteitsbibliotheek Gent, BIB.TH.003654: GB
– Österreichische Nationalbibliothek, 4.D.25: ÖNB
– WWU Münster, Bibelmuseum, H 1550: Bibelmuseum
– Regensburg, Staatliche Bibliothek, 999/2Script.67: MDZ
– [provenance unclear]: CSNTM (pages with biblical text only; good resolution; no navigation)
– Universiteitsbibliotheek Gent, BIB.ACC.016001: GB (vol. 1); GB (vol. 2)
– Bibliothèque de Genève, Bb 2299: e-rara
– Assisi, Pro civitate museum, Cinquecentina 13-1 and 13-2: PCM (vol. 1); PCM (vol. 2)
– Augsburg, Staats- und Stadtbibliothek, Th B IV 17: MDZ (vol 1); MDZ (vol. 2)
1. 1546
Τῆς καινῆς διαθήκης ἅπαντα. Novum Testamentum. Ex bibliotheca regia (Paris: Robertus Stephanus).– Österreichische Nationalbibliothek, 1.L.29: ÖNB | GB
– Regensburg, Staatliche Bibliothek, 999/Script.7: MDZ
2. 1549
Τῆς καινῆς διαθήκης ἅπαντα. Novum Testamentum. Ex bibliotheca regia (Paris: Robertus Stephanus).– Österreichische Nationalbibliothek, 1.L.34: ÖNB
– Universiteitsbibliotheek Gent, BIB.TH.003654: GB
3. 1550
Τῆς καινῆς διαθήκης ἅπαντα. Novum Iesu Christi D. N. Testamentum. Ex bibliotheca regia (Paris: Robertus Stephanus, 1550).– Österreichische Nationalbibliothek, 4.D.25: ÖNB
– WWU Münster, Bibelmuseum, H 1550: Bibelmuseum
– Regensburg, Staatliche Bibliothek, 999/2Script.67: MDZ
– [provenance unclear]: CSNTM (pages with biblical text only; good resolution; no navigation)
4. 1551
Ἅπαντα τῆς καινῆς διαθήκης. Novum Iesu Christi D. N. Testamentum. Cum duplici interpretatione, D. Erasmi, et Veteris interpretis: Harmonia item Euangelica, et copioso indice (Geneva: Robertus Stephanus, 1551).– Universiteitsbibliotheek Gent, BIB.ACC.016001: GB (vol. 1); GB (vol. 2)
– Bibliothèque de Genève, Bb 2299: e-rara
– Assisi, Pro civitate museum, Cinquecentina 13-1 and 13-2: PCM (vol. 1); PCM (vol. 2)
– Augsburg, Staats- und Stadtbibliothek, Th B IV 17: MDZ (vol 1); MDZ (vol. 2)