The office of Thai Immigration in Sakon Nakhon is listed below with a map to the immigration office. This immigration office can assist you extend your Thai visa such as a tourist visa and also a Thai O-A visa or marriage visa for Thailand. See their telephone number below with their email address.
Note that you can apply for your visa extension anywhere in Thailand however it has to be where you live as you have to give them a map to your house or apartment in Thailand. Each immigration office has its own way of the visa extension and the visa extension process listed on this website is the way Thai Immigration in Pattaya does it. The sequence may be different for each local immigration office.
Some want a medical certificate for a Thai marriage visa while others don't as this is normally for a Thai O-A visa and not a Thai O visa. Check with the local immigration office in the area where you are going to be living what their procedures are for the extension.
Thai Immigration in Sakon Nakhon