If you have tried all you mentioned above then go to Naz Store near Sofital or Al-Shahwani, Arab bank round about, You can ask them from Artist's shops 'Al-Khattat' near Souq Najadah...
If you're looking for a bigger one then try some shops of interior decoration.
Once I had bought a poster from 'Al-Manahil' a book store in Mansoorah...