It would be a quick answer IF you'd bothered to mention what stamp you entered on ๐ฎ : as in visa exempt OR with a valid tourist visa from the consulate before you came here ๐
In Bangkok if you entered on a valid tourist visa entry (meaning you bought a visa from the thai consulate before you came here and were stamped in for 60 days) you go to the main immigration office in the government complex (Building B) out on Chaengwattana Road.
That location is here
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Where as IF you entered on a visa exempt entry <- showed up and got stamped in for FREE for 45 days) you go to the immigration office at IT Square Laksi Plaza, third floor
That location is here
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There are copy/photo shops at both locations near the immigration offices.
Keep in mind all immigration offices in bangkok are closed until 8:30am monday morning because of the APEC Summit.