Where can I get a tourist visa in Bangkok if I’m already in Thailand with a visa exemption and an extension?

You can’t get a visa whilst already in the country if you have used your extension you will have to leave the country and return on either visa exempt or spend a few days away and apply for the visa at an embassy. Just be aware some borders require you to spend atleast a day or two before returning, some you can literally just leave turn around and walk back through.

Also if applying for the visa some embassies require an appointment and you can’t just turn up to apply 😁

Tourist visa's can not be obtained in Thailand.

You will need to leave Thailand before your current extension expires. You can then either apply for a tourist visa at an embassy in a nearby country for 60 days or just re-enter visa exempt for 30 days. Both extendable by another 30 days

The simple conclusion is: no, you cannot apply for a second tourism extension on one entry stamp on a visa-exempt or tourist visa. You need to leave.

30 days tourism extension is available only ONCE per tourism entry.

As you entered on the visa-exempt, you are supposed to go to IT Square Laksi Plaza temporary office.

On the other hand, IF you entered on an actual tourist visa which you applied for and got before travelling to Thailand and were stamped in for 60 days, you will go to Chaengwattana Immigration office, Government Complex Building B for 30 days tourism extension.