Hey there, if you're talking about George Buchanan's De Iure Regni Apud Scotos Dialogus, there's a very good modern critical translation (Mason, Roger A and Smith, Martin S. A Dialogue on the Law of Kingship Among the Scots: A Critical Edition and Translation of George Buchanan's De Iure Regni Apud Scotos Dialogus. Burlington: Ashgate Publishing, 2004). A good university library will likely have it, and if not, try your interlibrary loan program.
As for why it's difficult to find, it's simply that the text is a bit understudied, because people aren't quite sure what to make of Buchanan. The other Calvinist resistance thinkers, like Mornay/Languet, Althusius, Beza, etc., are better studied (and this is my opinion, but the introduction to the Mason edition supports this view) because they fit neatly into a pattern and form a continuous tradition with logical leaps in their respective works that make sense. They come at the problem of resistance from an Old Testament and Roman law background; and especially in the use of the Old Testament as explicit historical precedent.
Buchanan does not fit neatly into this school of thought. He's often lumped in with the other monarchomachs (a term originally applied to these thinkers by the Scottish writer William Barclay in the late 16th century), but I think that this is done erroneously, and I'll explain my thinking. It's an understandable assumption (he corresponded actively with the authors of the Vindiciae, Contra Tyrannos and the Scottish edition of that text, also published in 1579, was actually attributed to Buchanan), but he actually approaches the issues raised by the monarchomachs in profoundly different ways, even if he reaches similar conclusions.
The big difference, really, is that where the monarchomachs approached the questions of resistance and rightful rule from a legal-historical perspective, Buchanan came at it from a humanist-philosophical angle that owed much more to Ciceronian pietas and civic virtue than it did to Justinian's Digest or the Old Testament.
In addition to this, he does not treat the Bible as a final and always-applicable source of authority. Whereas the other authors were content to quote-mine II Chronicles, Kings and Samuel to postulate the existence of a covenant between the rulers and the ruled, Buchanan took a step back and argued that "it is necessary to consider not only his words, but also when he wrote them, to whom, and why," in effect contextualizing the Bible to rob it of its contemporary political significance. Where the monarchomachs argue that the king's power is limited and resistance is justified because of a double covenant between God, the king, and the people, Buchanan argues that the king's power is limited and that resistance is permitted because that's how things are done in Scotland, and furthermore it aligns with Aristotelian concepts of the origins of sovereignty, with Stoic morals, and with classical(y-inspired) civic humanism.
I would argue that Buchanan, if he has to be put into any sort of ideological box, belongs more to the native Scottish tradition of historical-constitutionalism pioneered by his former teacher John Mair earlier in the sixteenth century, that stressed by historical precedent and Aristotelian naturalism that Scotland had always been a 'constitutional' monarchy that was governed with the consent of the people (or at least the representatives of the people).
So that's my roundabout way of saying why I think Buchanan is hard to find: because people haven't really known what to do with him. Traditionally, he's been lumped in with the monarchomachs, (e.g. William Dunning's article, The Monarchomachs, in a 1904 Political Science Quarterly) but as I've very briefly outlined above, he doesn't really belong with them though he comes to similar conclusions about the rights of the king. With all that having been said, Mason's edition is excellent and is really the best modern translation: the old Arrowood is simply awful by comparison (Quentin Skinner notes in his Foundations of Modern Political Thought, 1978, while discussing De Iure Regni that the existing translations were so bad that he felt compelled to do his own).
If you're interested in looking up these other texts:
Vindiciae Contra Tyrannos has a similarly excellent modern edition (1994), edited by George Garnett, from Cambridge University Press.
Mair's History can be found here
edit: stray words