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This is where you need to go in Bangkok , building 7 I think....I can check as I went there last week.
They will only provide a clearance if you have a contract for the job you are going to. Take a copy of it. You also need to photocopy every page of your passport that has a stamp on it. They can send you the certificate in the post for 160THB or you can go back and collect it for 100THB. Its taking about 2.5weeks to process at the moment.
You have to provide some additional info when you get there, but its pretty straight forward. I can't remember what time the office opens, but get there before 15.30 as it shuts around 16.30ish...I was there around 45mins last week