I understand that Luther and others translated the Old Testament into German from the Hebrew Text. How was the Hebrew text accessible/available to them in the 16th century?
Daniel Bomberg (c. 1483 – c. 1549) was one of the most important printers of Hebrew books. A Christian who employed rabbis, scholars and apostates in his Venice publishing house, Bomberg printed the first Mikraot Gdolot (Rabbinic Bible) and the first complete Babylonian and Jerusalem Talmuds.
This timeline shows that Daniel Bomberg's Rabbinic Bible containing the first printed Hebrew version (Masoretic text) was published in 1517. Martin Luther translated the New Testament in 1522. Bomberg printed a second edition Masoretic text prepared by Jacob ben Chayim in 1526.
So, printed copies of the MT were available before Luther's translation of the Old Testament in 1536.
This article from the University of Nebraska affirms that Reformers had in their personal possession copies of Blomberg's printed edition of the Biblia Rabbinica, which contained the Masoretic Text, Masoretic notes, Aramaic Targum and Jewish Biblical commentaries.
Bomberg also reprinted the Hebrew text of the 1517 Rabbinic Bible in quarto versions during 1517, in 1521, and in a revised form, reflecting to some extent changes made in the 1525 Rabbinic Bible, in 1525–1528. These smaller Hebrew Bibles were a popular, less expensive alternative to Rabbinic Bibles among Christian customers. Ulrich Zwingli owned a Bomberg quarto Hebrew Bible, as did Wittenbergers Johann Agricola and perhaps Martin Luther.
The above quotation is supported by the footnote
Herbert Migsch, “Noch einmal: Huldreich Zwinglis hebräische Bibel,” Zwingliana, vol. 36, 2009, pp. 41-48. Agricola’s copy is on display in the Luther Halle in Wittenberg. On Luther’s purported copy, see T. H. Darlow and H. F. Moule, Historical Catalogue of the Printed Editions of Holy Scripture in the British and Foreign Bible Society, vol. 2, part 1: Polyglots and languages other than English, Martino, Mansfield Center, CT, 2005, pp. 705, no. 5086.
The article also makes the point that further evidence suggesting that Luther had access to this work is Luther's clear and critical knowledge of Jewish commentaries on the Targum, which was included in Bloomberg's printed Rabbinic Bible.