I have an new Emerson thermostat and would like to connect a c wire so it always has power. I have an extra wire that's not being used, but I don't have a C wire terminal on my furnace. I'm wondering if I can use any of the unused terminals or if I can tap into one of the wires coming off of the 24v transformer.
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.
The B terminal should be you common wire. It looks like you have air conditioning, one wire goes to the Y terminal and the other is common. Often marked B for heat pump applications.
It's literally the same as the blue wire on the transformer as in JPhi1618 pointed out. Either spot will work the same. The B terminal is easier.