How do I extend a Single Entry 60-day Tourist Visa in Thailand, and where should I go?

Thanks for the heads-up. But that information is not completed.

In Bangkok,

IF you purchased a Tourist Visa before you travelled to Thailand and were stamped for 60 days on TR entry (as you did), you need to go to the Chaengwattana Immigration office, Government Complex Building B for 30 days tourism extension.

BUT

IF you entered Thailand on VISA-EXEMP for free for 30 days without getting Tourist Visa in advance, you need to go to the office at IT Square Laksi Plaza for 30 days tourism extension.