@Anaeh *******
that is between you and the company you are going to work for.
IF you have enough time on your current stamp AND the company you will work for can meet the requirements INSIDE the country you can get the Non-B visa at the immigration office.
IF you don't have the required time on your current stamp or your company can't meet the requirements inside the country then you take paperwork to a thai consulate in a nearby country and get a 90 day single entry Non-B visa.