I can't find any for sale by doing a quick google search. I lucked out and got this one in a grab bag of sewing notions I bought on ebay. I did find a newspaper ad for it though - they were sold in 1977 for $2.99.
Here's how it works. You take off the presser foot from the sewing machine and take out the needle. Attach the ripper to the machine (one hook goe where the presser foot was and one over the screw from the needle). The ripper juts out toward you. Then, when you run the sewing machine and the needle bar goes up and down, the curved blade also goes up and down (see photos 5&6). You now have 2 hands free to hold the fabric with the seam centered in the little "v" and the ripper will do all of the work.
I actually tried it and it works. It doesn't work so great on sweater knits, mostly because of the stretch of the yarn and looseness of the knit (you have to hold the seam taut). But, it does work great on woven fabric. You would probably only want to use it on big projects, because otherwise why go through the hassle of dismantling your machine.