How can I transition from a Non Imm-B visa to a 90-day visa after my work contract ends?

What immigration office do you use?

Thanks, you ask a letter of termination of contract at your employer, (to give yourself travel times, ask to take a date a few dates later then you actually stop working), you take this letter to the Ministry of Labor and ask them to cancel your work permit at the last day mentioned in the letter. You take the documents from this ministry and the letter to the Immigration Office and ask them to cancel your Extension of Stay on the same last day mentioned in that letter.

Before or on that last date you leave the country and can apply for a new visa, seems to me it will become a Single Entry Tourist Visa which gives you 60 days on entry back into Thailand.

I guess you have a similar stamp like the one on the bottom, not a non Immigrant B visa but as line 2 of the text Extension of Stay permitted up to .. /../.. ?

What kind of visa or Extension of Stay are you using at the moment and do you have a work permit?

Do you mean a 60 day tourist visa with a 30 day extension?

You should also tell us your age and nationally, and your prior entry history here.