Adding a C (common) wire to my Goodman (Janitrol) furnace
You are looking in the right place

The cluster of blue and green wires at the transformer is the correct place to tap off of using a piggyback tab (quick disconnect) terminal (Amazon link provided as an example only, they are fairly widely available through electronics parts suppliers).  Make sure to crimp it to the new wire using an appropriate crimp tool though (pliers will produce bad results).