Where can I extend my 45-day visa on arrival in Bangkok?

I think it's funny when people argue over semantics. Yes you should be able to extend your entry visa it at the immigration office and add another 30 days. Don't do it until your visa is close to running out though.

I think you mean you're on 45 day entry stamp, as there's no such thing as a 45 day visa. You extend at IT square Laksi

there's no such animal as a 45 day tourist visa on arrival 😛

How about we get your terminology dialed in AND tell you where to go for your extension?

If you showed up here without buying a visa from a thai consulate in your country before you came here and were stamped in FREE for 45 days you didn't get in on a tourist visa, or a visa on arrival, in fact you didn't get in on ANY visa at all. You just got a free entry stamp and that's called a "visa exempt entry" 🙂

To extend that stamp the one time for 30 days in bangkok you go to the Immigration office on the 3rd floor at IT Square Plaza Laksi at this location;

Its not a tourist visa.

You didn't get any visa on arrival.

You entered through the visa exemption that is 45 days..

You entered Thailand VISA EXEMPT... 45 day stamp.

Go to IT Square Laksi Plaza Thai Immigration office for a 30 day extension of stay 1900 Thai baht.

These are the common found requirements for the 30 days extension