http://www.lincolnworld.com/products/
I've ordered Lincoln's boon and should have it in a week or so. It looks nicely designed from what I saw on his DVD, so I live in hope.
I think it was Iain on another site that mentioned Piatt's Scarab, sold by Ronjo's. I've tried that too, and it's nice, but for some reason it's the only such device that's ever been 'spotted' (thankfully only by my wife on a practice run). I think it might have something to do with its metallic sheen.
Vernet's boon is okay. I use them quite often, but keep losing them. I'm banking on the idea that the Lincoln boon will be more cost-effective in the long-run, as it has its own measures in place to reduce the risk of loss.
As with Lady of Mystery, I rarely find a TT that fits snugly, and so a TT writer wouldn't work well for me. I did find a junior-size one at Dude That's Cool Magic. It's a better fit than most, but the lead itself is very messy, and I can't think of a practical way to conceal it about my person without getting covered in graphite.
And as for the under-nail type, I always seem to find that my nails are either too short or filed to the wrong shape for a good fit, and haven't got the patience to treat my nails like bonsai...
Pete