Dont know if there are any still about, but I bought a "Shadow" pick up about ten years ago and have used it ever since on my various Gold Tones and others. It included a replacement bridge with two tiny microphones built in and connected to a volume control which clamped to the rim of the banjo.
Otherwise, I have used a small condensor mike which was OK thro' my own PA but encountered problems when plugged thro' a "professional" set up.
Hope this is some help