How can I switch from a non-B visa to a tourist visa after my work permit expires in Thailand?

For both case you're going to need to cancel your work permit and work extension before you convert or leave.

This means you'll need to get documentation from your work stating your last day of work and taking that to cancel your work permit, then take that same letter and your cancelled work permit to your immigration office and get your extension cancelled. If you do not cancel your extension, even if you leave Thailand, you will be unable to get another long-term visa or extension until you do so, and will be charged 500 baht per day (maximum 20,000 baht) for failing to do so.

If you find a new job you would cancel your extension and apply for the new extension at the same time, so you just go from one extension to the next.

If you do not find a job then you can go about a week before the date on your paperwork and get everything cancelled, and immigration will stamp you back in until the date on your paperwork so you have time to leave. You would then leave and when you return you would get visa exempt (not visa on arrival) for 30 days if you return without a visa.