Where to connect C wire on older furnace
Connect the blue C wire to the junction of the thin white wire and the thick dark wires

If you follow the yellow Y wire from the thermostat to where it goes in your furnance, it goes to a thin red wire.  Since we know this wire is what energizes the coil of your A/C's compressor contactor during cooling, we know that the cable with the red and white wires in it goes off to the A/C, and that the white wire in that cable must connect to C in order for the A/C to work.  From this, we can determine that you can connect your blue C wire for your thermostat to the junction of the thin white wire with the two thick, dark wires; note that you may need to upsize the wirenut at that junction to fit everything into it.