The best 2 places in Qatar to get plywood & other stuff related,. you can go to the industrial area street #18,. plenty of workshops provide a wide range of plys with all thicknesses & different materials & qualities,. you will find also other basic building materials such as reinforcement steel & ties.

the other place is in Najma,. Souq Al-Haraj. but you will find stuff slightly more expensive & you will need to do some negotiations in case you're taking a sum of plys.

I do prefer the industrial area ;)