The ferrite toroids (beads) used to be available from RST in Grass Valley,
CA (the avionics kit folks). However, it's been quite a long time since I
bought the ones I used in my airplane (probably about '91 or '92) so don't
know if they are still a source. I think they still advertise in the ad
section of the EAA magazine (though not positive), so give it a look.

Good luck,

Dan Schaefer