You cannot buy the gadget by itself--you have to sign up for a membership to get the gadget--although you could try to 3D print one at your local library.
I recently gave up on SightRight. My brain never adjusted after many months of doing the drills. To hit a straight in shot, it looked like I was cutting the object ball to hit one diamond to the right of the pocket. It's pretty strange to cut a straight in shot like that and watch it go straight in the pocket. Instead of my brain adjusting to see a one-diamond-to-the-right-of-the pocket cut shot as straight in, I had to mentally adjust my aiming knowing that a one-diamond-to-the-right-of-the-pocket cut shot was really straight in--for instance if I aimed straight at the object ball, I knew that I would cut the object ball to the left.
In addition, cuts from right to left looked different to me than cuts from left to right, and I thought the whole point of aligning your vision center with the shot line was so that cuts from right to left looked the SAME as cuts from left to right.
The recommended stance never worked for me: I'm right handed, left eye dominant and my vision center is directly under my left eye, and to crane my neck to get my left eye over the cue, it felt like I was falling over to my right--not to mention the intermittent neck pain I experienced. I tried emailing SightRight for some advice, and they told me to schedule a lesson with Stephen Feeney.
Finally, I never could figure out how to get the SightRight gadget aligned with a pocket without using a laser, which was too much of a hassle. For instance, if you are at the pool hall and you pull the gadget out of your case and put it on the table to practice your alignment, how do you precisely place the gadget on the table so that it's pointing at the heart of the pocket? I think you need your own table, then you need to draw a line on the cloth using a laser, then you can just whip out the gadget and put it on the pre-drawn line.
I hope you guys have better luck.