If I wanted to search a planet's cities for a commerce center, how would I do it?

Go there and look.

Or hire someone to look for you.

Or try asking the group that controls the planet.

Thanks. I was hoping there would be an easier way, but that works too.

Commerce centres specifically are operating by trading groups. Most groups keep a list on their website. For example CSS's list: http://www.centrepointstation.com/services/markets.php might hold your answer.

Check through the sites of all trading factions if it's not on that specific list.