Unlike for St Thomas, most of Scotus' writings have not been translated into English. None of his most important theological works have been translated in full. The situation is better for St Bonaventure, but there is still no translation of his Sentences in book form comparable to the excellent and relatively cheap translation of the Summa theologiae by the Dominican Fathers.
That being said, one can go a pretty long way towards studying Scotus in English. This is mostly thanks to the labors of the late Fr Allan Wolter, who was a one-man Scotus publishing powerhouse. Wolter has published both anthologies of excerpts and some complete works, sometimes with commentary and sometimes without. These are the books I would recommend most highly:
Duns Scotus, Metaphysician, published by Purdue. Anthology of long excerpts from different works. Lots of commentary, covers a number of Scotus' most unique or famous arguments and positions.
A Treatise on God as First Principle, published by Franciscan Herald Press. The first complete work by Scotus you want to read, concerning proofs for the existence and attributes of God. Contains probably the most metaphysically complex and sophisticated proof of God's existence ever. My edition, the second (1982) has a very full commentary.
Questions on the Metaphysics of Aristotle by John Duns Scotus, published by Franciscan Institute Press. Read this to test your manhood: it's two volumes of 600 pages each, not for the faint of heart. No commentary. A very impressive, very confusing, very stimulating, very difficult work. I spent a summer trudging through the whole thing (before my Latin was good enough to read the original) and wrote my M.A. thesis on a little bit of it.
A Treatise on Potency and Act, also by Franciscan Institute Press. This consists of Book Nine of the Questions on the Metaphysics just cited, but with commentary and other helps.
All of the above (except the complete Metaphysics Questions) include the Latin with the English, which may come in handy if your Latin is so-so or if you plan on learning it. To really study Scotus or any scholastic, of course, you should learn Latin well enough to not need a translation. If it's any consolation, it takes significantly less effort to learn to read Thomas or Bonaventure than to read Virgil or Livy. Scotus is somewhat of a different matter because his Latin is weird and abstruse and difficult. Of course he's abstruse and difficult in English; there's no getting around it. But he's not syntactically complex or using a huge vocabulary like the classics.
I should mention that Wolter's commentary is not always very helpful. I remember it being pretty good in the "Metaphysician" volume, so-so in the "First Principle" volume (sometimes very illuminating and sometimes baffling), and completely useless in the "Potency and Act" volume.
Also necessary to mention is the Wolter-Bychkov edition and translation of Scotus' Reportatio I-A, also put out by Franciscan Institute, which is in two huge volumes. I didn't mention these above because I haven't read all the way through the first volume and don't have the second, but if you really want to study Scotus' theology and you can't read the Ordinatio in Latin (or can't afford it or find it), you will probably want this.
So there you have it. Just shell out a few hundred bucks, give it a couple years of onerous study, and you can be a Scotist too! While you're at it, learn Latin, dredge up another thousand from somewhere, and buy the Opera Philosophica and what's been produced so far of the Vatican edition! Then, if there's any water left in the well, send some of the good stuff over to us at The Smithy. I'm still missing a couple of volumes.
