What are the requirements for obtaining a spouse visa in Thailand if my wife is working there?

I'd say get the single entry 90 day Non-Immigrant type O visa based on staying with a foreign spouse who legally works in thailand.

Your wife will need to provide copies of her passport, visa, extension, work permit, employment contract along with the marriage certificate,

BUT

if you can get that visa before you wing your way here you will be way ahead of the game when you go to get the extension to stay based on marriage to a foreigner legally working here

Contact your embassy and see if you can get that visa and what the requirements are..

Worst case, get at least a single entry tourist visa and come in on that, then begin the process inside the country.