How To Get a Work Permit in Thailand
All foreigners who intend to work in Thailand have to apply for a work permit before they start working. The issuance of work permits falls within the competences of the Ministry of Labor. Section 8 of the Alien Business Act states that the future employer may apply for this work permit on behalf of the foreigner, but the work permit itself can only be issued once the foreigner has entered Thailand in accordance with the immigration laws and presented himself to receive the permit. Moreover, a work permit will not be issued to the foreigner who does not have a Non-immigrant B visa.
In practice this means that, If you found a job before you actually move to Thailand , the Thai company should send you the documents required for the Non-immigrant b visa. This visa can (only) be issued at the Thai Embassy thus this saves you a trip to a foreign country after having arrived in Thailand. You can enter Thailand with this Non-immigrant B visa and stay in the country for 90 days. Within that period you should collect your work should for which, again, multiple company documents should be provided. Having received your work permit you can apply for a one-year business visa.
If you entered Thailand with a tourist visa or Non-Immigrant O visa (accompanying spouse) and then find work, you should apply for a Non-Immigrant B visa before requesting a work permit. This means you should first apply for the visa with a Thai Embassies in Singapore, Kuala Lumpur, Hong Kong, Laos are used to receive such requests B visa the next day in the afternoon. Afterwards you can apply for the work permit with the Ministry of Labor, and then request the extensions as explained here above.