Maybe an option for you is to go to Party Kingdom on Al Khaleej street, nearby the Jaidah Flyover.

This topic (party items/decorations/costumes) has been discuseed here before, please follow the link for some iformtion already on the site (did you try searching before posting?)

http://www.qatarliving.com/search?cx=008937172676595728955%3A066rihafv68...

Regards!!