I once asked the same question and got many responses on how to build one for under $10 or buy a similar product on eBay.
I haven't used mine in a while but saw positive replys when I made my initial post. My problem is I tend to get steep on my downswing, so I thought this would be the cure. I video taped my swing and even though I didn't hit the arm, I did still came down a little steep. Somehow I still got away with it?