Did a little digging and it looks like the Book of Jeshar as referenced in the Bible doesn’t exist anymore! I copied and pasted some information below:

There is a book called “The Book of Jasher” today, although it is not the same book as mentioned in the Old Testament. It is an eighteenth-century forgery that alleges to be a translation of the “lost” Book of Jasher by Alcuin, an eighth-century English scholar. There is also a more recent book titled “The Book of Jashar” by science fiction and fantasy writer Benjamin Rosenbaum. This book is a complete work of fiction.

Another book by this same name, called by many “Pseudo-Jasher,” while written in Hebrew, is also not the “Book of Jasher” mentioned in Scripture. It is a book of Jewish legends from the creation to the conquest of Canaan under Joshua, but scholars hold that it did not exist before A.D. 1625. In addition, there are several other theological works by Jewish rabbis and scholars called “Sefer ha Yashar,” but none of these claim to be the original Book of Jasher.

In the end, we must conclude that the Book of Jasher mentioned in the Bible was lost and has not survived to modern times. All we really know about it is found in the two Scripture quotations mentioned earlier. The other books by that title are mere fictions or Jewish moral treatises.

Read more: http://www.gotquestions.org/book-of-Jasher.html#ixzz2wyTUlMlE

Another link to a longer, more detailed article: http://www.christianfaith.com/lookout/the-book-of-jasher

These seem like reputable sites (much more so than some of the ones claiming it is real), but one never knows with the Internet!