Where can I buy some formal trousers and shirts in Guangzhou?

You can buy them in the Tianhecheng Department Store (Beijing Road Branch) at no.238, 1st street, Beijing Road, Yuexiu District. There are also some branches scattering in different place.
Tianhecheng Department Store (Tianhe Road Branch): no.208, Tianhe Road
Tianhecheng Department Store (Fanyu Branch): Yingbin Road, Fanyu District
You may also consider the Liuhua Clothes Wholesale Market which is located at no.24 – 26, Zhannan Road, near the main railway station. Because of its location, this market may be a little crowed.