No C terminal on furnace to connect a wifi thermostat
The blue wire coming off of the transformer is what you want, but there might be a better place to tie into that.  Look for any wires that are simply screwed to the metal chassis of the air handler.  You can check with a continuity tester, but those chassis grounds are usually a connection for that other side of the transformer (the blue wire).

Looking at that schematic, I can see a few places where a wire terminates to "ground", but I don't necessarily see any terminals where the "C wire" would be conveniently exposed.  Also, when you find a ground screw use a volt meter to make sure you get a good 24v there between the ground point and the red wire from the thermostat and/or transformer.