If you were stamped for 30 days only, You are going to go to IT Square Laksi Plaza temporary office, just next to Lak Si station of SRT Dark Red Line.
You will go here.
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And you can book online here
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General information for 30 days tourism extension is here. Bangkok doesn't ask for a TM30 receipt for a standard tourism extension usually.
Photo & copy shops are just around the immigration office, fyi.