tareq -
I buy my lightroom software directly from Adobe, you can buy the
software by going to Adobe.com.
Click on MENU>PRODUCTS>ALL PRODUCTS>PHOTOSHOP LIGHTROOM>BUY (in the
lower right corner).
There are two different offers, one which is a subscription service
for photographers that includes Photoshop CC, Lightroom and Lightroom
mobile to which I subscribe at a monthly fee, and another which is
just Lightroom stand alone that you can buy outright. You have to be
careful that you do not buy the UPGRADE as you have to have an
existing previous copy of Lightroom because you will need the Serial
Number from that copy in addition to the current version of Lightroom
(which is 5.6).
I hope that helps!
Steve