Ferrite cores are carried by the major electronics distributors such as Mouser and Digi-Key. You can get split cores that can be snapped around an existing cable / conductor or plain cores where you have to thread the conductor through them. Effectiveness can be increased by making multiple turns of the conductor through the core. You probably want to select one that's effective around common com frequencies like 122.8 MHz. Make sure the hole in the ferrite will accommodate the conductors you want to pass through it. For the ammeter application, you might want to pass both sense leads from the shunt through the ferrite.

You'll have to sort out minimum order and shipping prices.

Here's a link to one datasheet: http://www.mouser.com/ds/2/212/KEM_E...-SR-372794.pdf

Note: I don't have any noise issues on my Model III and haven't used these in aircraft service.