This is the third branch of the prasthana-traya - you will need a specialist bookstore to locate it. Note that, since you will presumably only be interested in the Advaitic interpretation, you will want the one with the bhashya by Shankara.
The most popular version is probably
the one with commentary by Swami Gambhirananda
(available from Vedic Books).
It is an exceedingly difficult book to
read and, though it contains some of the
most profound philosophical analysis, it
is certainly not for the beginner.
Rather than a direct translation, this
version is paraphrased by the Advaita scholar
V. H. Date and is much more readable. Unless
you really want the most accurate rendition,
this is probably the best choice.
Both versions contain Devanagari for the
sutras themselves. You may currently buy
it (in two volumes) from Vedic
Books.
