Where should I go for a 30-day extension on a visa exemption in Thailand?

You can also use the newer E-extension and get your extension on the third floor at Chaeng Watthana, but it is much more expensive, at bt1900 for the extension, which is the same cost, but then a minimum of a bt500 processing fee, up to bt5000 depending on how fast you need the extension. Not many are aware of this E-extension service yet, and for both visa exempt, and TR60 entries. Tourists come under category 2.4.

TR Bangkok Immigration Bureau / Por.30 IT Square Laksi Plaza, Bangkok

For visa exempt you would go to IT Square Laksi.

If you had entered on an actual tourist visa you purchased from a Thai embassy, that's when you would go to Chaeng Watthana.