Where would be the best place to get transponder circuits?

Electronic stores, maybe?

You can build them with a level 2 electronics skill out of some signal recievers and wire IIRC, or I believe you can also disassemble two-way radios or teleporters to get them.

Thanks for the tip/

Indeed. Easiest way by far is to disassemble radios, or loot electronics stores and disassemble the junk you acquire in there.

Yeah, I managed to get quite a bit out of the electronics store in the town outside my base.