JR Buddy Boxes
I'm no authority on this... but I believe that you can use another 421 transmitter for a buddy box.
I just taught a student flyer at our field a while back and they had a JR 421 transmitter. No one had a buddy box so I called Horizon Hobby and asked them what they recommended. They recommended the JR Quattro ( which is what they sent me, minus a crystal ). It's a good basic radio that includes the trainer cord. No servos. I think I paid $50 for it.
I hope this helps!
Regards,
Mike