OBNB doesn't sell books, but you may be able to find a copy at one of these websites:
Kitāb al-irshād = The book of guidance into the lives of the twelve Imams by Muḥammad ibn Muḥammad Shaykh al-Mufīd. ISBN 0907794009. Published by Balagha in 1981. Publication and catalogue information, links to buy online and reader comments.