I will be travelling Guangzhou for the first time. I'd like to know where to buy a street map.

Many places in this city sell the street map. Don't worry.
After arriving, you can buy it at the airport or railway stations.
In the downtown area, you can find several newspaper stands and bookstores along streets. Most of them sell the map.

As a native Cantonese, honestly I don't think a paper street map can help.

A better way would be knowing your destination from google first, then find out the best metro station. But google is not working in China. So you'd better got it prepared before your trip, or you'll need a VPN go google something.