DON'T BE SAD book is a great book Mash'Allah. My brother bought it for me, it's a constant reminder for all that we DO have, that's the main theme.
Anyway, as to where to purchase it, you can get it from all good Islamic Bookshops. All the one's I've been to, tend to stock it. Also, you can probably get it online. It's quite well known.
Anyway, don't know where you're based.. But here's the publishers details.
Title: DON'T BE SAD
Author: 'Aaidh ibn Abdullah al-Qarni
Publishers: International Islamic Publishing House (IIPH)
Editor: Yusuf Riyaz
Insh'Allah this will help you in your quest to find this book.