No C terminal on furnace. Workarounds? Possible the AC?
In your pictures, there is a 24vac transformer:



It has a red wire and a green wire coming off the bottom of it (wire colors are more apparent in the other pictures).  The green wire then gets grounded to the case of the furnace.  This is the common wire.  In the top left of my picture, there is also a blue wire attached to the case.  This is also the common wire.  Blue is the typical color for common, so splicing into that blue wire (it most likely already has a wire attached to it somewhere that goes to the A/C unit) is the best thing to do.

Note - not all blue wires are "the common wire".  The particular blue wire I pointed out is, but don't assume that all blue wires are ok to connect to.