ebay, they show up from time to time, get out your checkbook though. You could build one, its not rocket science, just mimic the v way. Done on a milling machine, a novice ought to be able to bang one out in a few hours. Probably lots of you tube videos on building one.
I have a dial indicator with a 2" travel and a magnetic back that I put on the right wing of the carriage in back. I use a small welder's right-angle magnet from Harbor Freight to hold a block of steel on the bed for the tip of the indicator to operate against. It's no problem stopping the carriage when under power feed just by watching the motion of the needle.