Old AC and Gas Boiler wiring for new thermostat
You want the C terminal in the air handler, yes

The terminal you're looking for is indeed the air handler's C terminal -- it's on the fan-delay board (in the back left, from the viewpoint of the picture you've posted).  You'll want to tap the connection it's making to the blue wire using a "piggyback" tab terminal or by cutting and splicing into the blue wire connected to it with a wirenut.