You can find very good quality boxes in Q - Post (different sizes). All info in their site :

http://www.qatarposts.com/gpc/service.htm#Parcel%20Posting%20Service.

You may want to call them 4464446 to find out where they sell them. I used them in a previous moving and since them I re-used them twice and they are still alive :-)

The following box sizes (cartoons) are available:

Box Size in cm. Price View Image
32.00 x 31.50 x 03.00 QR. 02.00 Sample
32.00 x 31.50 x 05.50 QR. 03.00 Sample
32.00 x 31.50 x 09.00 QR. 04.00 Sample
10.00 x 25.00 x 17.00 QR. 02.00 Sample
12.50 x 34.50 x 17.00 QR. 04.00 Sample
24.50 x 39.50 x 24.50 QR. 05.00 Sample
19.00 x 49.00 x 28.00 QR. 06.00 Sample
40.00 x 50.00 x 45.00 QR. 07.00 Sample
50.00 x 60.50 x 50.00 QR. 10.00 Sample
60.00 x 60.00 x 60.00 QR. 12.00 Sample
50.00 x 100.00 x 60.00 QR. 14.00 Sample